I don't understand it myself, Alan. I literally cannot hear anything in my head. The reason I know this is because on maybe five or six occasions, I have heard something in my head (just a fleeting thing like a voice lasting only a second or less), and I know what that's like. But those were freak occurances for me, and I don't understand why they happened then, or why I can't hear in my head the rest of the time.

I can hum a tune or sing a tune (or more often, in my case, whistle a tune) from memory. It's just that my memory doesn't come in the form of hearing in my head. I don't know how to describe what I experience... it's more like a feeling about a sound rather than something that is experienced like a sound. I have the same problem with seeing in my head. It's totally dark and quiet in there pretty much all of the time. But I am very kinesthetic, and I have "feeling-senses" for most things. I know that is probably very difficult to imagine if you're not like that, but I don't know how else to put it.

And also, although I know what notes to play in a piece that I have memorised, I really don't "remember" them as such. I just know which notes to play automatically without thinking about them at all. This is the case for me with timing as well.

Oddly enough, as I have said, I am not a very good sight reader. In fact, I am a very poor sight reader. Most of the time, I need to hear a piece of music a few times before I can use sheet music to learn it. I need to hear the timing and the melody before I can understand how it's supposed to sound. Then I use the sheet music to help me memorise it. Once I have memorised a tune, I will sometimes need to continue to use the sheet music for the chords for a much longer time before I really "know" them and can play the piece without having the chords on marked on the sheet music in front of me.

Knowing the chords is entirely muscle memory for me. I don't know how they're supposed to sound until I play them, and I only know which ones to play because my muscles have memorised them for me (I play accordion, so it's simply a matter of pushing a button to get a chord). This is not at all the same thing as the way I learn a tune.

I have some other interesting brain wiring quirks as well. I had some repeated head traumas as a child (ironically enough, while falling off of the family piano as a toddler), and I think it's possible that this may be the reason for the way my brain works (or doesn't, as the case may be).