I'm posting this just because it was so weird, in the hopes that maybe it helps someone somewhere down the road.
Earlier today, videos embedded in web pages just stopped playing on my laptop. E.g., if I go to YouTube and click on any video at all, it won't start. Or, better, the ad won't. It just kind of sits there with this spinning thingy and in some cases pops up a dialog saying "If playback doesn't begin shortly, try restarting your device".
Same problem with Chrome, Firefox, and Konqueror. Same problem in both KDE and XFCE. Same problem even with a new user. Some kind of system problem, apparently.
The web suggested disabling hardware accelaration in Chrome, but that didn't help. It turned out that the problem was a mis-configured audio device. I'd somehow managed to put the internal audio card into "Analog Output Stereo" instead of "Analog Output Duplex". Setting it back unblocked the device somehow and allowed the videos to play. Why that didn't affect videos playing in VLC, say, I don't know.
Thanks to
https://www.techdim.com/fixed-if-playback-doesnt-begin-shortly-try-restartin...
for the clue.
Riki