> I'm not sure everyone has this ability, but whenever I've written/recorded music, I almost literally hear it before I even try to create it or play it. My memory of sound is REALLY strong

I’m the same. Details in sound is the same as details in vision for me, I can remember the fine details of complete songs seemingly forever (yet my memory for everything else is terrible). And I have the same thing where I have the sound and musical progression of what I’m about to make in my head, ‘playing’, before I even start trying to make it happen on a synthesiser or on my modular system. And I don’t just mean known sounds like violin, piano, etc. I’m doing sound design as well as composition in my mind (I can hear it) before I start doing it for real.

I can easily believe that Beethoven was capable of “hallucinating” the music that he was composing in the same way.

Interestingly, I don’t really get this when reading sheet music, only when I’m creating or reflecting. Presumably, it’s a different pathway in the brain.