In "Sonny's Blues," is music a savior in Sonny's life or his ultimate weakness?James Baldwin's "Sonny's Blues"

Maybe both.  It is a known fact that musicians live an odd life, sleeping during the day and working at night.  Their schedule and their association with "goodtime people," as Sonny's father had called them, can often lead to drug use.  Added to this temptation in "Sonny's Blues" is the isolation Sonny feels when he tells his brother that he wants to play jazz, and the narrator-brother is not too receptive to the idea.

When Sonny stays at Isabel's house before she and the narrator are married, living with Sonny is difficult as he plays notes over and over:

Isabel finally confessed that it wasn't like living with a person at all, it was like living with sound.  And the sound didn't make sense to...any of them....It was as if he were all wrapped up in some cloud, some fire, some vision all his own; and there wasn't any way to reach him.

This inability of the family to reach Sonny is what probably precipitates his estrangement and drug use:

For he also had to see that his presence, that music, which was life or death to him, had been torture for them and that they had endured it, not at all for his sake, but only for mine.  And Sonny couldn't take that.

Then, years later, the brother reads of "Sonny's trouble."  After Sonny comes to live with his brother, Sonny invites the brother to join him as he plays that night. And, it is in this night that Sonny's life is saved, for his brother--sitting at a small table in the dark--sees and hears the suffering in the man that is his brother Sonny for the first time:

...the man who creates the music is hearing something else, is dealing wit the roar rising from and void and imposing order on it as it hits the air.  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 tale of how we suffer, and how we are delighted, and how we may triumph is never new, it always must be heard.  There isn't any other tale to tell, it's the only light we've got in all this darkness.

Sonny's blues/jazz is this light in the darkness; it is his savior.  The brother-narrator describes the glass that the waitress puts on top of the piano for Sonny as glowing and shaking "above my brother's head like the very cup of trembling," the life-blood of the Savior.