Update: I played more video/audio with my venerable, very good, Plex Media Player and it all continues to work perfectly. Conversely, the new Plex player not only does not play audio, as described, but, additionally, stutters the video, skips video frames and just does a horrible job.
Result: Sticking with my Plex Media Player … perhaps someday they’ll fix the bugs in the new Plex player and I’ll switch over ---- until then I’m not using the new Plex player at all.