"But the man who creates the music is hearing something else, is dealing with the roar rising from the void . What is this "void"in Sonny's Blues

In James Baldwin's "Sonny's Blues," I don't know if we can
be certain of what the "void" is, but we can speculate, as we might with any kind of
art: narratives, poems, music, paintings, etc. My impression of this phrase is the
Sonny's brother, as he is listening to his brother play, realizes that there is
something the his brother hears that most do not.

The
speaker says…

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All I know about music is that not many people
ever really hear it. And even then, on the rare
 occasions when something opens within,
and the music enters, what we mainly hear, or
 hear corroborated, are personal, private,
vanishing evocations.

With
this statement, the narrator puts most in the category of people who hear music and just
don't "get" it, or get close, but experience it only as it pertains to them personally.
Sonny is different. Hearing music transports him; it changes his world, which other than
music, is pretty unimpressive. He doesn't hear what everyone else hears. When he
listens, he's...

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...dealing with the roar rising from the void and
imposing
 order on it as it hits the
air.

When the music hits
Sonny in it's pure and muddled—Sonny works with it, though the speaker says that it
isn't easy for him, but it is good for him,
and for us.

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What is evoked in him, then, is of another
order, more terrible
 because it has no words, and triumphant, too, for that same
reason. And his triumph, when 
he triumphs, is
ours.

The
void could refer to a place deep within that may speak of a
darkness in the soul—what exists within a person when no one else is around. Or it could
speak to the chaos that rests within a person that can be controlled only by music: as
if the "music soothes the savage beast." For as the void, the chaos, the darkness hits
the air, the music takes the madness and forces it to take shape and make sense, which
is what happens when Sonny plays his
music.

It is one of the few things Sonny can "control" in
his life, and one of the few things that brings him real pleasure and a sense of
accomplishment.