You know, not theme tunes or songs written for the soundtrack, but songs that fictional bands perform in the universe the film or show exists in. They're all a bit the musical equivalent of this picture, aren't they?
Link me to some. I have soft spots for the following.
Low Shoulder's 'Through The Trees' from Jennifer's Body: http://www.youtube.com/watch?v=d1riVmHVteY
Dean Geyer & Caitlin Stasey'S 'Unforgettable' from Neighbours: http://www.youtube.com/watch?v=CgYVYkZIQ8Y