OK, this happened on Sunday. I fixed it, so I thought I'd report it here in case anyone else runs into the same problem.
I have a Forerunner 945 (obviously), connected to a Samsung Galaxy Note 9 and AfterShokz Titanium headset. I've loaded several albums of music onto my watch. (Basically, the first sixteen albums of the Progarchives top 100 studio albums.) So I set it to play songs randomly while I was running.
Nothing. No sound. No music. (I did hear the headset turn on and announce that it was connected.)
This was the first time I had used this setup, and I couldn't figure out what was going wrong. Max volume on the watch. Max volume on the headset. Nothing.
What I didn't think about was that the phone was set to silent. No announcements and no media. That overrode the settings of my watch and my headset.
Today it works fine after I unmuted my phone. Maybe I'll finally be able to run with music tomorrow morning.