Why do we say that the sound "AUM" is identical with Brahman?
This sound "AUM" is a combination of three syllables:

(1)'Aa'
(2)'Oo' and
(3)'M'.

'Aa' is connected with awareness. It is called 'Viswa 'OO' is the subtle and is connected with ideas of the mind, with dream. It is being considered as 'Taijasa'.

'M' is causual, is connected with cause and effect and is related to
deep sleep or 'Prajna".

So when the sound 'Aa' joins with 'OO' and when the sound 'OO' joins with 'M' we get the complete sound (AUM). That is when the 'Visva' merges into 'Taijasa', and 'Taijsa' into 'Prajna', we get the darshana
of Atma. In the same way it is only when we are able to combine
i.e. the three states, the waking state, the dream state and the deep sleep
stage, or combine the gross, the subtle and the causal aspects
into one, we have a chance of getting a glimpse of Divine Soul.

Thus AUM, 'the Pranava', represents the entire content of Brahman.

-Sri Sathya Sai Baba.