Mandukya Upaniṣada

aomityetadakṣaramidam sarvam, tasyopavyākhyānam,
bhūtam bhavatbhaviṣyaditi sarvamomkāra eva;
yaccānyat trikālātītam tadapyomkāra eva (Mandukya Upanishad 1)

In the preface mentioned sound om is everything - aomityetadakṣara idam sarvam
The past, the present and the future, (the whole description of the material worlds) is certainly the all-embracing sound "om" - tasyopavyākhyānam bhūtam bhavatbhaviṣyaditi eva sarvam kāra om
Whatever is different from these three times -yad anya trikālātītam,
that is also the sound "om" - tad api eva omkāra.

sarvam hyeted brahmāyamātmā brahma
so'yamātmā catuṣyāt (Mandukya Upanishad 2)"

Everything comes from the spiritual energy - sarvam hyetad brahma
the spiritual energy also contains the souls - brahma ayama atmā
These souls exist in four (states of existence) - so'yamātmā ṣyāt catu. 

jāgaritasthāno bahihprajñah saptāṅga
ekonavimśatimukhah sthūlabhug vaiśvānarah
prathamah pādah (Mandukya Upanishad 3)

The first state (of souls) is the external consciousness - prathama pāda bahi prajña
The waking state common to all people- jāgarita sthāna vaiśvānarah
as an enjoyer of the gross elements- sthūlabhug
with seven limbs and nineteen mouths - saptānga ekonavimśatimukhah

To clarify that behind the sound Om is the Supreme Lord,
Sri Krishna says, as recorded in Bhagavad-gita 7.8,

raso 'ham apsu kaunteya prabhāsmi śaśi-sūryayoḥ
praṇavaḥ sarva-vedeṣu śabdaḥ khe pauruṣaḿ nṛṣu

O son of Kunti, I am the taste of water - kaunteya aham rasaḥ apsu
the light of the moon and the sun - prabhā śaśi sūryayoḥ
I am the sound Om in all the Vedas - asmi praṇavaḥ sarva vedeṣu
the sound (quality) of space- śabdaḥ khe
and the skillfulness in people - pauruṣam nṛṣu.

svapnasthāno'ntah-prajñah saptāGga ekonavimsatimukhah
praviviktabhuk taijaso dvītyah pādah (Mandukya Upanishad 4)

The second category (state) is the inner consciousness- dvītyah pādah antah prajña
in the fiery dream state - svapnāsthano taijasa
grounded in enjoyment - pravivikta bhuk
with seven limbs and nineteen mouths - saptānga ekonavimśatimukhah

In this context it is said in Brihad-Aranyaka Upanishad 4.3.18:

tad yathā mahā-matsya ubhe kūle 'nusañcarati
pūrvaṇa cāparañ caivam evāyaṁ puruṣa etāv ubhāv antāv
anusañcarati svapnāntañ ca buddhāṇtañ ca

Just as a large fish moves to both shores - tad yathā mahā-matsya anusañcarati ubhe kūle,
also the enjoying (souls), lying (in the body of Viṣṇu) behind before - evāyaṁ puruṣa apara pūrvaṇ
come to the boundaries between the two - etāv caivam antav ubhāv
wandering between the boundaries of dream and waking state- anusañcarati svapnāntañ buddhāṇtañ.

Bṛhad-Araṇyaka Upaniṣada (4.3.9) explains that the two shores are the material and spiritual worlds.
In these circumstances, the "river" in between means the Causal Ocean,
where the souls are placed by the Lord and choose between the two kinds of worlds:

• the dream-world - the material world
• the world of the waked souls - the spiritual world.

This "free" choice of souls will bring some souls to the material worlds.
This would be the so-called "original fall", or the seed of all future sins,
for thus one has chosen to play the "enjoyer"
and not that of the (direct) servant (of the Lord), which is in itself a higher enjoyment.

tasya vā etasya puruṣasya dve eva sthāne /
bhavata idaṇ ca paraloka-sthānaṇ ca
sandhyaṁ tṛtīyaṁ svapna-sthānaṁ /
tasmin sandhye sthāne tiṣṭhan ete ubhe
sthāne paśyatīdaṇ ca paraloka-sthānaṇ ca (Bṛhad-Araṇyaka Upaniṣada (4.3.9)

There are certainly two places which the enjoyers (souls) reach - dwe eva sthAne etasya puruSasya bhavata
This (material world) and the highest (spiritual) place - idaṇ ca paraloka-sthānaṇ
There is a third position, which is a dream place - tṛtīyaṁ svapna-sthāna,
the junction (between the two) - sandhyaṁ.
Residing at this connecting place - tasmin sandhye sthāne tiṣṭhan
Seeing (the souls) both (worlds) - ete paśyati ubhe
This (material world) and the highest (spiritual) abode- idan paraloka-sthānaṇ.

So the Iskcons-Prabupada's idea that the souls would fall from the spiritual world down in the material world is false.

In the 5th verse of Mandukya Upanishad, the state of uninterrupted consciousness is described:

yatra supto na kañcana kāmam kāmayate
na kañcana svapnam paśyati tat suśuptam
suśuptasthāna ekībhūtah prajñān-ghana evānandamayo
hyānandabhuk cetomukhah prājñas-tritīyah pādah (Mandukya Upanishad 5)

The third category (state) of consciousness - prājñas tritīyah pādah
the uninterrupted consciousness is the state of sleep - prajñāna-ghana yatra supto
without longing, (without) golden-shining desires - na kāmayate kañcana kāmam
one does not see golden-shining dreams - na paśyati kañcana svapnam
that is the deep sleep state) - tat suśuptam.
In deep sleep situated in one being (with the Lord) - suśuptasthāna ekibhūtah
from comprehensive ecstasy, one is ecstatic - evanandamayo hyānandabhuk
and one satisfies the mouth of consciousness - cetomukhaḥ.

Some interpretations of this verse say that one wanders in deep sleep in the body of the Supreme Lord and thus becomes one with the Lord - ekibhūtah and enjoys an ecstatic state - ananda, 
but there is no confirmation of this speculatioan in the Vedic literature.

In this case it is certain that the souls who have not attained liberation and are not qualified to have a direct relationship with the Supreme Lord in the spiritual world, at the destruction of the material worlds, they are immerse themselves in the body of the Lord and wait there in this third state of unbroken consciousness - prajñāna-ghana, until the next creation, when they are thrown again in the material existence.

In the next creation, these souls are transferred to the material worlds, but they are preloaded with the desires and reactions that remained unspent in the previous creation.

An explanation of this verse can be found in Bhagavat Puran 7.9.32 when the famous devotee Prahlad spoke to the Supreme Lord Nrisimha -deva,

nyasyedam ātmani jagad vilayāmbu-madhye
śeṣetmanā nija-sukhānubhavo nirīhaḥ
yogena mīlita dṛgātmani pīta-nidras
turye sthito na tu tamo na guṇāḿś ca yuńkṣe

(O, Lord) With  -closed eyes, absorbed in Yourself - mīlitadadṛg ātmanipīta
situated in the transcendental sleep state (like a waking sleep)-yoga nidra turye sthito
but not in the darkness of material modes - na tu tamo guṇā
you joined the (liquid of) destruction with the dormant inactive soul within you - yuńkṣe śeṣetmanā ātmani nija nirīha
(who) had experienced the bliss (of ecstasy) - sukhānubhavo
by throwing them into the center (of the waters) of destruction (and creation) (the Causal Ocean) of the universes - vilayāmbu-madhye jagad nyasyedam

The sixth verse is an explanation of the previous verse:

eśa sarveśvara eśa sarvajña esho'ntaryāmyeśa
yonih sarvasya prabhavāpyayau hi bhūtānaM (6)

This is the Lord, the All-Controlling One this is the Lord the All-Knowing One - esha sarveśvara eshha sarvajña
This is the Lord, the indwelling controller (the Oversoul) - esho'ntaryāmyesha
the source of all (existing), the beginning and the end of all living beings - yonih sarvasya prabhavāpyayau hi bhūtānaM.

It is an explanation of the previous verse, where the unbroken consciousness of the living entities was depicted as entering into the body of the Supreme Lord after universal annihilation and becoming "one being" with the Lord, and before being directly transplanted into the material worlds due to their previous activities and unfulfilled desires.

Only the pure souls are transplanted in the intermediate worlds (the Causal Ocean), and accordingly have the choice between the material and spiritual worlds.

The 7th verse of Madhukya Upanishad describes the 4th state:

yah nāntah-prajñam, na bahiprajñam, no'bhayatah-prajñam na
prajñānaghanam, na prajñaṁ, nāprajñaṁ
adriṣṭamavyavahāryam agrahyamalakṣanam-acintyam
avyapadeśyam-ekātmapratyayasāram prapañcopaśam
śāntam, śivam-advaitam caturtham manyante sa ātmā sa vijñe

The fourth (state) is the soul known (in itself) - caturtham sa ātmā manyante
It should be realized. - sa vijñe
Neither the inner consciousness nor the outer consciousness - nāntah-prajñam na bahiprajñam
neither fearless consciousness nor uninterrupted consciousness - no'bhayatah-prajñam na prajñanaghanam
neither consciousness nor non-consciousness - na prajñam, na-aprajñam
(The soul is) invisible, indescribable, beyond direct - perception, without (material) qualities, inconceivable - adriṣhtam avyavahāryam agrahyam alakshanam acintyam
indefinable, the essence of which is the soul itself alone - avyapadeśyam eka atma pratyaya sāram
the end of material existence, - prapañca upaśam
peaceful, blissful, non-dual - śāntam, śivam-advaitam.

This fourth state is to be realized and not the other three.

In this context, Śuka Gosvamī says, as recorded in Bhagavat Puran 2.10.6:

nirodho 'syānuśayanam ātmanaḥ saha śaktibhiḥ
muktir hitvānyathā rūpaḿ sva-rūpeṇa vyavasthitiḥ

After the (cyclic) annihilation (of the material worlds) - asyānuśayanam nirodha
the souls are liberated with the (material) energy. -ātmanaḥsahaśaktibhiḥ muktir.
Getting rid of his other form - -hitva anyathā rūpaḿ
(the soul) is in its own (spiritual) form. -vyavasthitiḥ sva-rūpeṇa

This own spiritual form should be realized, and this corresponds to the fourth state of consciousness.

In the third state - prajñāna-ghana - the soul is immersed in the body of the Lord without attaining liberation, and only in the fourth state attains liberation and also its own spiritual form - sva-rūpeṇa

The next verse (8) explains what is the difference between the mystic sound 'om' and its components ('a', 'u' and 'm') in relation to the four states of consciousness.
OM is the full extent, which describes the full consciousness of realization as a soul,
when one has realized one's own spiritual form - sva-rupa,
whereas the decomposed form of the "om" sound into its constituent parts ("a", "u" and "m") describes the other 3 states of consciousness as explained in the verses 9-11.

so'yamātmā-adhyakṣaram-omkāro'dhimātram pādā mātrā,
mātrāsca pādā akāra ukāro makāra iti (Mandukya Upanishad 8)

This soul beyond the alphabet sounds - sah atmā yama adhy akṣaram
Is the mystic sound "OM" beyond all elements - om-kāra adhi mātram

The full extent of all states - pādā mātrā
And this full extent also manifests itself in this way - mātrāsca pādā iti
the sound "a", the sound "u" and the sound "m" ("aum") - a-kāra u-kāro ma-kāra

jāgaritasthāno vaiśvānaro'kārah prathamā
mātrā''pterādima tvādvāpnoti ha vai
sarvān kāmānādizca bhavati ya evam veda (Verse 9)

The first element (state of the soul) in the material world, - ha āpterādima mātrā
the waking state common to all people - jāgaritasthāno vaiśvānara
is the first sound "a" - a-kārah prathamā
by which one attains the state of duality - āpnoti vai tvādva
and one realizes all primeval desires. - kāmānādisca bhavati ya
This is knowledge. - evam veda

svapnasthānastaijasa ukāro dvitīya
mātrotkarṣādubhayatvādvotkarṣati ha vai 
jñana-santatim, samānasca bhavat
nāsyābrahmavit kule bhavati ya evam veda (Verse 10 )

The second sound "u" is the fierry dream state - u-kāro dvitīya svapnasthānastaijasa
of a higher quality. - -mātrotkarṣā
It refers to both and is also the place of both: spiritual knowledge and peace - ad ubhaya tva dva jñana-santatim
Ascending to the state of equanimity - utkarṣati bhavat samānas ca
(at this level of consciousness) one is no longer born in the class of those who do not know spirituality. - na bhavati asya abrahmavit kule
This is knowledge. - evam veda

suṣuptasthānah prājño makārastṛtīya mātrā
miterapīter vā, minoti ha vā idam
sarvam-apītisca bhavati, ya evam veda. (Verse 11 )

The third sound "m" is the consciousness of deep sleep. - tṛtīya ma-kāras prājña suṣupta sthānah
like the state of a fish in the water; - mātrā mina uti ha
neither immersed in it, nor in this world (it remains only to be in the Lord's body) - apita va vā idam
And just like that (from the body of the Lord) everything comes into existence. - apī iti sarvam bhavati
This is knowledge. - evam veda

amātrascaturtho'vyavahāryah prapañcopaśamah sivo'dvaita
evamomkāra ātmaiva, samviśatyātmanātmanam 
ya evam veda, ya evam veda. (Verse 12 )

The fourth state is soundless and motionless, - caturtha amātras vyavahārya
glorious, non-dual, the end of the materially manifesting. - siva advaita prapañca upaśamah
So the sound "om" is really the soul - evam om-kāra eva ātma
together with the Lord, the soul of all souls. - sam ya iśa atmanātmanam
This is real knowledge, this is real knowledge. - ya evam veda, ya evam veda

So there is a connection between the perception of a transcendental sound and the consciousness of the speaker / listener.

A self-realized personality, a pure devotee of the Lord, when he contemplates in his purity and is not "touched" by the world when he utters the Holy Name of the Transcendental Lord, the Lord reveals Himself to the devotee in the form contained in that Name. The consciousness is at the level of the soul, and the sound is at the para level.

When one thus chants the Hare Krishna mantra, the Lord reveals Himself as Krishna.
The deeper the relationship with the Lord becomes, the more parts of His pastimes are revealed;
in the beginning one sees from the outside, and later one is in there.

At this level, rebirth (in the material world) is impossible.

The Power of Sound

In Tantric writings the sounds of the Sanskrit alphabet (deva-nagari) also called akashara have corresponding guardian deities also called Shaktis (energies).

Akshara are sounds that have a certain force, they are coupled with the phases of Creation (material worlds).
Akshara as defined by Vedic Knowledge are constitutionally coupled with the objects they name by the resultant sound (shabdha) and its meaning (artha).
The sounds of the current languages are not these Akshara or stilted variants of them.

Transferring transcendental sound to the material plane

Baladev Vidyabhuasan wrote in his commentary on Vedanta Sutra 1.3.28, that the creation of all beings was continued by the first being in a universe, Brahmā by remembering their forms and characteristics by uttering the corresponding words.

This is another level of understanding of the power of words.
The eternity aspect of the sound of śabdha and the meaning (artha), it carries with it, is based on their spiritual origin - śabda-artha-brahma, as the Transcendental Lord  is the Origin of objects as well as their description (functionality).

Just as a Sankalpa, a vow or thought, must pass through several stages to attain the expected purity in order to manifest as a creative Force, so the sounds of the mantra of initiation must pass through several stages to attain the manifesting power of the mantra's guardian deity.

The stages that the Sound goes through are called para, pashyanti, madhyama and vaikhari.

In the para stage the devotee purely utters the name of the Supreme Lord Krishna and this becomes manifested through the power of the mantra

The initiation mantra is not a random combination of sounds but the subtle form of the Deity that is present in the Mantra. Continuous meditation on the spiritual name present in the Mantra leads to purification of thought and sound.

The 4 steps of sound

Each stage of sound corresponds to a certain stage of existence (consciousness ), and the perception of a certain stage of sound depends on the stage of purity of one's consciousness.

To perceive the full spectrum of sound (not in the physical sense) requires a pure, spiritual consciousness. Seers who can perceive all four stages of sound are called Maniṣi.

The three higher forms of sound are called in the Vedas:
guha, meaning hidden, or lying within the self (to which people conditioned by the modes of material nature have no access), while the fourth stage is known as manifested language - laukika bhașa.

The four stages of sound correspond to the four stages of consciousness:

Para is the transcendental step, corresponding to spiritual consciousness - of pure goodness - chuddha sattva.
Pasyanti is the material step corresponding to discriminative consciousness, the stage in which intelligence - buddhi, keeps the mind under control, corresponding to existence in Virtue - sattva gun.

Madhyama is the material stage corresponding to mental consciousness, the mental step - mana, corresponding to Passionate existence - raja gun.

Vaikhari corresponds to gross materialistic consciousness - corresponding to Ignorant, dark existence - tama gun.

The mantras received in the initiation process have the following purposes
to purify the mind of material attachment
and also to create the mental link
with the form of the Absolute Truth upon which one meditates
during mantra utterance.
Man-tra means liberation (tra - the Sanskrit word root for trayate) and mana means mind,
that is, the liberation of the mind from its preoccupation with material matters (of this world).