Wait - there are people who can't do this? How do they get ear-worms? If you imagine Boris Karlof singing "You're a mean one, Mr. Grinch", and the voice in your head doesn't sound like Boris Karlof, what does it sound like? How can you do a Ronald Reagan impression if you can't hear what Ronald Reagan sounds like in your head?

I get terrible, terrible ear-worms. I once heard parts of the first 2 movements of Beethoven's 5th nonstop for almost a week.

I've introspected about this a lot - yes, introspection bad - trying to figure out how many parts I can hear at once. At first I thought I could hear 3 to 4 parts at once (4 only when the song was very familiar or the parts were very different). But I can't hear even 2 parts begin at precisely the same moment. It seems to require very rapid, barely-perceptible, attentional switching between parts, on the order of tens of milliseconds, to change the note.

Mozart could reproduce complex polyphony after hearing it once, so he must have been able to hear and imagine all the parts. Although I'm sure he had very good compression and predictive accuracy to help him reconstruct it.