The narrator makes these remarks in relation to the music played by his brother Sonny. What he appears to be saying is that there's something in the music that his brother hears but other people do not.
This is not an unusual observation. Our experiences of listening to music are always going to be different from those who compose or perform it. This is because the music comes from a place—a void, as the narrator calls it—deep within the soul of those who make music. As such, it has greater meaning for the composer or performer than it does for the listener.
The listener doesn't really hear the music—he or she only internalizes what relates to them personally—the music-maker is completely transformed by it. Sonny is a prime example of this. His ordinary world, his everyday, workaday world, really isn't much to write home about. But when he listens to the music that he plays, his whole world is transformed; he's transported to a different place entirely. What he hears is radically different to what we as listeners hear. It evokes in him a whole different reality, an order without words that he himself has created. The music gives him a sense of triumph that he simply can't find in a mundane, everyday existence marred by poverty, racism, and drug abuse.