I hear music almost continuously in my head and it can drive me nuts. Usually it's a single phrase, from a popular song or a TV commercial jingle, and it repeats over and over again. I can't stop it; I can only "replace" it by thinking about a different tune instead.
It's kind of like "chewing gum", in a way, i.e. it's in my head and my brain, my auditory cognition system, is idly chewing on it, sort of like how your mouth might chew on a wad of gum.
Listening to music engages the apparatus suddenly in "active" mode, like it's been idling waiting for a task to perform on and now it's ready to go. I do find that I listen to/hear music differently than nearly everyone I've ever spoken to about it. I group heard phrases in certain ways, because of this "chewing" mechanism in my head. I think it's a kind of "windowing" phenomenon as in the computer science concept of a sliding window on data.
I started to work out a system of algorithmic-style composition, material generation, based on this, on windows applied to superposition of rhythms, in my "LCM" series, beginning with my "LCM for 12 Piano Sample" on my CD and continuing with "LCM.4567 for 4 marimbas" ... I guess if I get bored enough one of these days I might try typing in another essay for y'all, about the LCM thing, since the response was positive to my previous essay about "The Rat's Nest".
Playing music, i.e. performing on an instrument, was one of the best ways to eliminate this repeating head music. I used to say that maybe the music pressure was building up in my head and I had to release it, or let off the steam, by playing, to keep my head from exploding :-)
But now I haven't played my instruments for a couple of years, and the repeating tune in my head, more often than not, is a fragment of some etude that I used to practice on the bass ....
Chris Koenigsberg firstname.lastname@example.org