Same here after Sonoma 14.4 upgrade. The "I opened a specific music file in Finder, it played in Music and it fixed that one file" comment was a big hint (h/t illbilly).
My files (>23k mp3s, etc.) are also in non-standard location. The following fixed it for me: YMMV, back up everything, cross your fingers, and don't blame me...
Referring to the "Files" pref in Music where the directory is set...
- Do a "reset", and say "yes" to subsequent questions about moving files. For me, this seemed to gather/process a few files, but not >23k.
- Change the directory back to the original (non-standard) file location. Again, say "yes" re: moving/keeping files organized. For me, the process went through all >23k files, didn't duplicate them on my local media, and retained everything in my Music library.
I'm thinking this stems from some security "update" in MacOS 14.4 where Music wasn't being granted permissions to access the media files. Something tells me that 14.4.1 is coming soon.
I agree with all here that the Music app is becoming more and more braindead, and the neat iTunes power mode features (like, ummm, song search by title only, so many other things), are sorely missed.
Good luck everyone with this issue and I hope this process provides the same success I had. Now, to try the same thing with the TV app (and probably Podcasts, where I know this is happening too).
EDIT: FWIW... same technique fixed files in TV for me, too. Podcasts seem to be kept in a MYSTERY LOCATION and was a nonstarter anyway as Apple is treating them as all cloud based.