Mixer Wierdness: Score produces no sound

I recently installed MS 4.5.1 on my M2 Mac Studio (MacOS 14.7.1) and got to work on a score I've been working on for the past year. Somehow I got it to a point where it no longer produces any sound - none of the level indicators in the mixer show anything, export to a WAV file produced 5 minutes of silence, etc. (It does export valid MIDI files for the parts.). Other scores I've got (which I haven't worked on recently) all play back as expected.

And I accidentally fixed it just now!

In testing other scores I noticed that in the misbehaving one the mixer's mute buttons were grayed out for all of the instrument parts except for one - a Claves part that I use as a metronome and which was explicitly muted. When I umnuted this part and clicked on its solo button, all of the other mute and solo buttons came to life! The result was - quite literally - music to my ears!

How I got into this state I don't know; is there a good place to post a workaround tip for others who might encounter this?