the song running in the background of one's mind as one goes about the day. used affectionately for the songs that persist for minutes or hours and that, like a movie soundtrack, reflect or amplify one's mood (rather than the odd, irrelevant songs that occasionally get stuck in one's head).
it's april, sunny, and getting warm. my mental soundtrack is, "oh what a beautiful morning."
she dumped me. my mental soundtrack is playing, "here he comes / that's cathy's clown."
that guy looks just like donald fagan. everytime i see him, my mental soundtrack switches to steely dan, "hey nineteen."