Sorry, I could have expressed myself better. I mean, music in a level is half OF THE LAYOUT of the level. Eyecandy is the visual part of it, music the auditive one. Personally I don't give the former more importance than the second, so a level with a music that has nothing to do with the level itself irritates me as much as a level with an awful background, for example.
Anyway, I give much more importance to things like gameplay or originality, than to eyecandy and music. But, as you correctly guessed, I'm talking about SP levels. I'm quite sure things in MP are a lot different.