In my opinion it's really hard to get a music theme going for Slaanesh. (S)he is the lord of excess and the only thing his followers need to do, is do what they like to do in the extremes. So if someone likes metal, keep turning it louder and louder and perhaps commiting atrocities while listening to his type of music? Trance is good for them ofcourse but I think you can't exclude other types of music. Khorne, rage, more metal \m/ Tzeentch perhaps some psychedelic music? Shpongle perhaps? Nurgle, I honestly wouldn't know what kind of music fits them.
Tzeentch I'm thinking Tubular Bells, Mike Oldfield, for some reason. Or Eye in the Sky, Alan Parsons Project... Nurgle? Tough... Carebears (daddy nurgle loves everyone)? ;-) But really, I'd say some form of Doom/Black/Sludge Metal, opposed to the Thrash/Heavy/Speed Metal of Khorne? For some reason I think this would fit pretty damn good:
Pretty easy. Slaanesh: Annoying dubstep. Tzeenetch: Probably electronica Nurgle: That's the hardest to figure out. Khorne: DEATH METAL HEAVY METAL ALL THE METAL.
