Don't know what I did, but they are working.  Computer off overnight.  Unplugged speakers for a couple of minutes and plugged them back in.  Plugged speaker cable into green socket on back of computer, got MaxxAudio popup, clicked OK and Windows found them.  Windows 11 is a fickle mistress.
Speakers plugged into green socket (on the back) are not detected.  Support Assist scan revealed NO hardware problems.  Windows (11) troubleshooter said "no speakers plugged in."  Speakers have a TRS plug. Speakers were working (yesterday, I think).  Updated BIOS in the last day or so; updated normally.  Drivers are up to date.  Only thing I've done today is plug in a new webcam (USB). Windows Control Panel > Sound does not recognize the speakers.  Does recognize my monitor speakers (HDMI). I've rebooted, removed the speaker plug and plugged it back in.  Nothing has helped.
XPS 8940, speakers in GREEN socket are not detected