I recognise myself in a lot of the replies in this thread. I have music playing in my head all the freaking time, usually the one song on repeat, or, worse, two lines from a song on repeat. It is annoying, but the annoyance level fluctuates depending on how distracted I am by other things.
Strangely, even though songs on repeat in my brain are a right pain, in the actual world outside my mind, I will develop an obsession with a certain song and listen to it on repeat until I get sick of it. I guess the difference is that at that point I can turn it off.