Verse 10

10.  Taijasa,  whose field is the dream state,  is the second sound,  U,  because this is an excellence,  and contains the qualities of the other two.  He who knows thus,  increases the flow of knowledge and becomes equal to all; in his family there will be born no one ignorant of Brahman.

"Taijasa,  whose field is the dream state,  is the second sound,  U,  because this is an excellence".

Taijasa has the meaning of luminous,  it is used to describe the individual lighting his own dream world.
Taijasa is Consciousness displaying the experience of the dream state illuminated through its own understanding and desires,
Consider also Brihadaranyaka Upanishad 4.  3.  9  :-
...by his own light and dreams.  In this state the man himself becomes the light.

After the creative quality contained in the all encompassing first sound  (A)  this,  "U"  has the knowledge to continue from that first sound and so maintain the progress of Creation,  it will also carry the knowledge of the last sound to enable Creation to pass to that stage.

"and contains the qualities of the other two".

As said,   "U;"  contains all the qualities  (Knowledge)  to maintain Creation.

"He who knows thus,  increases the flow of knowledge and becomes equal to all"

With reference to having all the qualities,
The quality of knowing the Absolute produces an equanimity with the creation of events as it unfolds.  This has the effect of allowing all knowledge to flow naturaly and unimpeded or altered through "individual desire,  through this knowledge produces its maximum possibilities.  That is,  "increases the flow of knowledge".

But,  "He who knows thus"  Also has the meaning of he who meditates or prays with the understanding of duality and desire.  It is he who only  "knows"  creation through desire.  Therefore this man will maintain his stature and presence within creation  "becomes equal to all"  through further desire and creation  which will also "increases the flow of knowledge".

"in his family there will be born no one ignorant of Brahman".

Brahman here refers to the lower conditioned Brahman of creation.  This man of good intention but who only understands creation in terms of duality understands Brahman through its material attributes.
This is the natural condition of the "householder" and father,  the unrealised man,  the creator of his own world.  It follows that this understanding of the householder will continue through his  "family" or line of dependants being born into this material realm.
Consider Brihadaranyaka Upanishad 4.  4.  22 :-
The ancient Sages did not desire children

In Taijasa man dreams and experiences the fruits of his desires.  Man also dreams of further desires or he tires and dreams of return to the Absolute.
This means that man either seeks further worldly knowledge or he seeks the higher knowledge of the Absolute that transcends the dream.

This  "dream state"  also has the meaning that,  because all the work required to make manifest the Universe was completed within the sound of  "A"  there is no work to do in this state.  Which makes all work and desire in this state an imagination or dream.