I hear music in my head all the time. I walk around all day hearing tunes. I also see in my mind's eye where my fingers will go on the fiddle fingerboard to play the tune. But I generally don't know or think about what actual notes I am playing. I can't hear harmonies in my head, nor sing them. I definitely think playing by written notes (which I used to do with woodwinds) and playing by ear involve opposite sides of the brain. In my case, the cross-communication between sides is kind of slow sometimes.