It seems as if one of Adorno's major concerns is that regarding who defines what music is. A thoroughgoing realist approach to universals would suggest a well-defined categorization that is discovered and revealed by some official body such as the church, nation state, or music "industry". A thoroughgoing nominalist approach, on the other hand, would lead to each musical piece being a one-off defined by the composer and each performance by the performer. Is Adorno aiming to seek some middle ground where the definition is not constrained by either an official body or a single artist? If no one sets the categories then you have randomness and white noise. So he must see some need for categorizations maybe controlled either by a community (not necessarily musical) or nature. Or maybe his search is ultimately futile.
I think what Adorno really wanted is for music to make you get out there and revolt against capitalism :3 . If you could bring on the revolution by classifying Barry Manilow as Death Metal, I'm sure he would've been down for that!
