Yep, I do this. In fact, sometimes when I'm meditating and focusing on my breath, I start hearing a song. I often ask, "Is that still being mindful or is it cheating?" I usually settle on cheating and my brain tries to focus in on the breath again. Sometimes I just let it play.

You know what's really weird? If I don't know all the lyrics to a song, I can sometimes listen to it in my head and hear the parts of the song I don't consciously remember.