Soundtrack in your head

I watched a DVD about Buffy Sainte-Marie last night. In it, she said that she always has music playing in her head, and that when she was a kid, she thought that everyone did.  That got me wondering -- what runs in my head?  And is it different from what runs in other people's heads?

Much of the time, in my head there are emails and blog posts being composed.  Sometimes I imagine scenarios of things that might happen in the future, like if some event that I fear or hope for occurs, how will I deal with it?

Also, when I hear music, I see dance in my head.  Not all music, but sometimes.