iTunes suddenly decided my mp3s are mpga movies that iTunes won't play. What happened?
Here's a workaround, which also makes no sense: as explained at this link, switch iTunes to run in 32 bit mode via the Info box.

This doesn't explain parts 1-4, but it does at least make the tracks playable. I'd still love to know what is actually going on.

Also, you can enable the "Kind" column and sort by it to discover which of your tracks have mysteriously been converted to Quicktime Movies.