I've had this too. When it's quite, usually in my apartment, I'll hear music. It's been different kinds; Irish fiddle tunes, Enya, others. Sometimes mine sound like they have words but it's not loud enough that I can make it out. At first, I thought maybe one of my neighbors was playing it. However, when it started getting personal (genres and songs I listen to), I knew it was in my head. I find it kind of cool.