The answer is quite simple. Harlem is a neighborhood that is afflicted with the dual diseases of poverty and racial discrimination--that is, it is home to African-Americans who are discriminated against.
Young men growing up in Harlem often feel that they have no hope of escaping the poverty of their neighborhood. So, they often turn to crime, or to drugs, as Sonny does. Sonny's obsession with music can also be linked to this hopelessness. It is quite possible that his interest in music is a way to escape from the problems of the neighborhood.