I have come to a very obvious conclusion. Soul Music is more of a Death book than a Susan book, for all it introduces her and focuses on her for half the story. The story structure focuses more on Death. I'm thinking particularly of the Misbegot Bridge where the beggars live.
Classic rock songs have a musical bridge, which is often instrumental though it can have words. It divides the song briefly and adds a different element. After the bridge, the song returns to its main theme but (ideally) the meaning has been affected by the bridge. The story in SM pauses when Death is under the bridge, hanging out with the beggars for a while, to forget and to have some human companionship that doesn't require a history or a memory - existence in the moment. Susan's story has a bridge when she stops following Buddy in order to visit the past. Buddy's story has a bridge when the band is on tour, though in a way, for him the entire book is a bridge. Even Albert has a bridge moment, when his lifetimer breaks and he is frozen in time. Once they are all back in A-M, the story continues, and in each case, they have been changed because of the bridge. The story begins and ends with Death, and Death's experience of a memory-less existence under a literal bridge allows him to forget, however briefly, until he is recalled to his memory by Buddy's song and by the Death of Rats.