I like the expression "ear candy" ("mind candy" is even better), but it seems to refer only to music that is easy and pleasant to listen to, but not music you can't get out of your head. So "Ohrwurm", ugly as it sounds, is probably too positive in its connotations to fit the gap of the word you were looking for to denote the state in which a tune won't stop playing in your head. Oh well.