I always have a song playing in the background of my mind. I can be singing the same song for a week, or it might change during the day. Sometimes it's just a song I heard that got caught in my head (I even catch the school recess bell melody if I'm not careful). Often it reflects my mood, state of mind, the thoughts I'm preoccupied with. Hearing a song can make me recall exactly how it felt inside my head the last time it was playing in there.
I'm going to use this blog to track the playlist in my head.