Huh, I thought *everyone* had music that played in their head on a regular basis. I know it's more rare for me NOT to have music running in my head than the other way around. The default tune seems to be the Star Wars overture. Yeah, full movie orchestration. It's the default tune, just the general muszak of my soul.
I hum along with what's in my head a lot, too. People have said that they can tell my mood by what I'm humming. I don't need to listen to external music. I have my own. When I write down "Current Music" for LJ, it's what is currently running in my head. Every now and then, I just run up against something that makes me realize that things I take for granted, as something all people experience, is relatively rare. It has made a lot of things in the world make more sense, when I make these realizations, though.