The fact is that I did try a few reboots along the way, but I think the bitclean I ran, followed by a reboot, is the thing that somehow reset it. One more of those unexplained things where you keep doing stuff until something works. Thanks again for your interest and help.
More really good ideas, thanks. No headphones available (gasp) but tried all else, checked all. Ran full scan w/MS Security Essentials, ran bitclean, will reboot but no change in spkr status yet, will system restore to point a few days ago later...
Only me at the keyboard, but you could say Google Chrome messed with it. I got a suspicious-looking notice that Chrome was back-level and click here to update. I thought it may have been misspelled, with a missing letter in the middle of the link, but stupidly thought it looked genuine...