Music (Apple app) won't play .m4a tracks from its own user interface, will play if I double click on the underlying file.

When I select a track using the Apple Music app, it either simply doesn't play, or raises a dialog "You are about to start playback. Do you want to clear the song previously add to Playing Next?" with 3 options "Clear Song", "Play Song" or "Cancel". Only Cancel does anything (closing the dialog). This happens whether I double click on the track, select the track and click on the > arrow at the top, or right click and select Play Next.

If instead, I find the file in Finder and double click on it, it opens Music and plays the track successfully most of the time. Sometimes it first raises the same dialog box. In Finder, the icon by the file changes from a generic icon with a music note in it to an icon with the album art on it. Once that has happened, the track will play without a problem when selected within the Music app, using the view I generally rely on, Artists.

If instead I find a song using Songs, double clicking results in Music playing the last song it played rather than the song I select that way.

What gives?

Music (the app - btw Apple, please name it something more distinctive) seems to have gone nuts. I have restarted the app, and the operating system, several times. I have unlocked the Music folder (using Get Info) - it has read/write permissions for me already, but (following a lead from elsewhere in Communities), I clicked on Apply to enclosed folders.

I have my iTunes library on Dropbox, but do not suspect that is the problem as I've had it there for years and this problem only emerged today.

Any suggestions for restoring sanity to Music? It is not available for reinstall via the App Store, being built in to the operating system.

Apple M1, Sonoma 14.4.1

Thanks for any clues.