ITunes suddenly decided the mp3s are mpga movies that iTunes won’t play. What happened

You can just replace the file in the filesystem.

Unfortunately, iTunes is too clever for it's own boots and registers for Finder events. So renaming a file in Finder will update it's location in iTunes if iTunes is open!

So here's one way I've tried that works.

1. Quit iTunes (actually quit, not just close window);
2. Locate the file in Finder:
   • This could be done in iTunes (before quitting) by right-clicking and choosing Show in Finder;
3. Replace the file in Finder with one with the exact same filename;
4. Open iTunes

iTunes will then use this file when you attempt to play it. Note: if any of the ID3 tags differ they will be updated in iTunes too (which is probably what you want). This means if you replace it with a file with the same file name but with less information in the metadata, you'll have less information in the iTunes browser.

You can delete items whose files have been removed by running this from AppleScript Editor:

tell application "iTunes"
    repeat 5 times
        repeat with t in file tracks of library playlist 1
            try
                if location of t is missing value then delete t
            end try
        end repeat
    end repeat
end tell

I don't know why you have to run it multiple times, but it usually removes all items after about five times.