To add more anecdotal evidence, I also "hear sounds" in my head that relate to music. I can catch myself actually processing these as audio which seems similar to your statement of actually hearing songs in your head. As soon as I notice it, it will go away.

The real example, however, is that I took an intro from a folk song and made it my ring tone. I hear that thing everywhere even when my phone is not ringing. I have no idea why. If I think, "was that my phone?" I start hearing the song.

Personally, I find it annoying, but not harmful.