From : Deussen,The System of Vedanta and others.

Vedanta is a spiritual tradition which is concerned with the self-realisation of the ultimate nature of reality (Brahman). It teaches that the believer's goal is to transcend the limitations of self-identity and realize one's unity with Brahman.

"Brahma satyam jagan mithya, jiva-brahma iva-na param" - the only reaity is Brahman (Advaita- Vedanta)

Advaita Vedanta was taught by Adi Sankara and his guru Gaudapada. According to this school, Brahman is the only reality and the material world is illusory.

Hinduism knows numerous interpretations of the "sutras" and of the "upanishades" by wise men who were more studied men than selfrealized masters (who seldom wrote big books in the past centuries).

Traditional hinduism distinguishes between a brahmin who posesses the abilities of a brahmin and a brahmin who only has the outer schooling of a brahmin.

In some interpretations the brahman is put equal to the atman.
Above the atman is ISHVARA and above all are the divine worlds. Often everything above the four lower worlds is normalized as "Brahman". Shankara teaches the four elements and the Akasha. Above all is the "Brahman".

The problem is that many authors give interpretaions of their personal view of things that happened on their personal path. But only a master of the step 20 of the UNIVERSAL PATH, the fifth initiation, can see the synthesis of things.
Surely NADA or Shabda exists indirectly in ALL beings of the 4 lower planes (which have MAYA is their basis).

The Prashna - Upanishad says : "It is true, o Satyakama, the sound AUM is the higher and the lower brahman".
Even A.A. Bailey admits that the "AUM"-Meditation" is more for beginners and has the small disadvantage that it stimulates the negative forces (U stands for the negative material Aksahas !)."

The transcendental vibration omkara is the sound form of the Supreme Personality of Godhead. All Vedic knowledge and this cosmic manifestation are produced from this sound representation of the Supreme Lord.
-- Chaitanya Charitamrita, Madhya-lila 6:174

The Vedic sound vibration omkara, the principle word in the Vedic literatures, is the basis of all Vedic vibrations. Therefore one should accept omkara as the sound representation of the Supreme Personality of Godhead and the reservoir of the cosmic manifestation.
-- Chaitanya Charitamrita, Adi-lila 7:174

Therefore the eplaination of "AGNI YOGA" about AUM is therefore wrong : "U" does not stand for light or for the second ray , etc..
This false interpretation of "U" also exists in some tibetan scools. "U" stands for the material AKASHAS , ie. - Maya and Karma ! Some tibetan lamas give the same false interpretation.

(10). Kala Hamsa, the "Bird" or Swan (Vide No. 11).
Says the Nada-Bindu Upanishad (Rig Veda) translated by the Kumbakonam, Theosoph. Society :
"The syllable A is considered to be its (the bird Hamsa's) right wing , U, its left, M , its tail , and the Ardha-matra (half metre) is said to be its head."

Ann.: But it must be said that "Hamsa" is the perishable goose and still not the white atmic swan, the atmic "Soham". So one must be careful: An Upanishad is just a secret teaching and not the absolute truth !

"Salutations to Omkara, which gives whatever one desires and also liberation to those who meditate always on Omkara that is united with the Bindu."(Guru Stotra)

In order to become the KNOWER of ALL SELF, you have first of SELF to be the knower.
" To reach the knowledge of that SELF, you have to give up Self to Non-Self, Being to Non-Being and then you can repose between the wings of the GREAT BIRD.
Aye, sweet is rest between the wings of that which is not born, nor dies, but is the AUM throughout eternal ages. Bestride the Bird of Life if you would know. Give up your life, if you will live.
(The voice of silence, H. Blavatsky)

OM is the supreme symbol of the Lord. OM is the whole, OM affirms; OM signals The chanting of the hymns from the Vedas. The priest begins with OM; spiritual teachers And their students commence with OM. The student who is established in OM Becomes united with the Lord of Love.
-Taittiriya Upanishad

A perfect path of the Truth has come into being for our journey to the other shore beyond the darkness. Rig Veda

AUM. This imperishable word is the universe. It is explained as the past, the present, the future; everything is the word AUM. Also whatever transcends threefold time is AUM. All here is God; this soul is God. This same soul is fourfold.(Mandyuka-Upanishad)

Wiki about the Mandyuka-Upanishad
Hinduwiki about the Mandyuka-Upanishad)
Wiki about the indian philosophy