Soul Music is the story of how rock n' roll came to Discworld and got everyone all shook up. It's also the story of Susan's first time stepping into her grandfather's professional shoes (her grandfather being the anthropomorphic personification of Death). Lots and lots of promise there, and Soul Music definitely doesn't fail to amuse. Rock n' roll, called Music with Rocks In for Discworld purposes because the drums are a set of rocks banged on by a troll, literally infects the residents of Ankh-Morpork from the Unseen University down. Suddenly seventy year old wizards are acting like teen-agers and fashioning robes out of studded leather, and middle-aged housekeepers are tossing their undergarments at the lead singer of The Band.
The lead singer is the connection between the Susan plot and the music plot, and it's a connection that I feel wasn't taken full advantage of. The singer, a young druid named Imp y Celyn, forms a band with a troll and a dwarf, accidentally gets his hands on a magically electric guitar, changes his name to Buddy, and unleashes Music with Rocks In on the unsuspecting populace. Unfortuantely, Buddy was supposed to have died, (enter Susan doing her grandad's job), but the music reanimates him. Her heart then goes "twang" and she spends the rest of the book trying to save him from the music. I think. It gets a little muddy in there. And that's the main problem.
Pratchett tosses lots of fun stuff at the reader, but ultimately, when it comes time to wrap everything up, it all feels rather messy and contrived. In the end, it does all come together, but it does so in a bit of a tangle, so while Soul Music is definitely a fun little ride, it leaves you in a less than satisfying place. I wouldn't go so far as to not recommend it to someone looking for a little light fun, but if you want an ending that satisfies, I might look somewhere else.