In the right circumstances and frame of mind, symphonies or sometimes brass band tunes play in my head, multilayered and everything. They've even got enough persistence that I can "rewind" them a few seconds (probably within some kind of working-memory window), even isolate parts of them and usually know exactly what instrument is playing it, and so on, then let them continue. The course of them is automatic, I don't control them, though.
If I had the first clue how to record them, perhaps I'd have a career as a composer, LOL. The actual invention of them would be no work whatsoever, though the writing it down would be, and I'm sure there'd be a good deal of editing and arranging afterwards to fix them up (plus, who's to say if they'd be any good, or wouldn't all sound kinda the same, to a trained ear?)
I'm only barely familiar with the body of "classical" music, and even less familiar with big-band or brass band music, is the oddest part, but those couple narrow sorts of instrumental music are all I get without having to put effort into it (and I mean none, it just "plays" when I'm in the right head-space and surroundings, and no I don't mean "on drugs", and actually it can be really fucking annoying if I'm trying to sleep). I wouldn't be surprised if I actually lost that ability (such as it is) if I tried to train up enough to write the tunes down.
... maybe I should look into humming-to-MIDI software, hahaha.