Verse 11

11.  Prajna with his field of activity in the deep sleep state,  is the third sound,  M,  because this is the measure,  and that into which all enters.  He who knows thus,  measures all and becomes all.

"Prajna with his field of activity in the deep sleep state,  is the third sound,  "M",

Prajna has the meaning of pure consciousness and knowledge.
Here Prajna is Creation returning to the wisdom of rest and stillness.  This Creation is said to be the result of the ignorance of desire.    Ignorance means the forgetting of the Existence and Non-Duality of the Absolute.  This ignorance resulted in the apparent duality of Creation which contains the imagining of incompleteness and need or desire. 
"M"  is the ending of  "AUM"  which is the ending of the error of Creation and ignorance and the returning to the unity and stillness of Absolute Knowledge as Consciousness Alone.

"because this is the measure,  and that into which all enters"

As said the  "M"  at the ending of  "AUM"  has the measuring quality of Tamas described by the "Gunas".  All the sounds in  "AUM"  and all of Creation merge into the Tamas and sleep of the  "M".

"He who knows thus,  measures all and becomes all".

He who knows this measure knows the cycle of creation.
He who knows the cycle of creation knows the Absolute.
He who knows,  he who Realises the Absolute,  becomes all as the Absolute is All,  and sees the way to transcend the creation cycle of birth and death.
M begins the end of creation.  M is the measure of all things.  Here all that is and was starts the return to rest.  This transient display of the eternal knowledge of the Absolute unwinds and once more becomes unmanifest,  formless,  without attributes.