Mind's Ear

I've wondered. When I think of a song and actually listen to it in my head am I actually hearing it or not? I mean, it's confined in my head, but is it possible to hear inside-out instead of outside-in? I guess the same principle would apply for mind's-eye and what not.

It just sounds so real to hear a whole song within the confines of my own mind.

I remember pondering that a long time ago, but I never really found an answer. I do the same thing when I read to, I hear the words that I see.