So instead of trial-and-error restoring each service I disabled, restarting after each and testing headphone sound, I just restored Windows 7 from a recent disk image instead (like a 1-day old image, so nothing was lost).
Headphone sound now works again, and I've learned my lesson about fiddling with services, for now at least. But fwiw I'm 99.999% certain this breakage was caused by me disabling a service that has some non-obvious connection to the headphone sound.
Fwiw here's the list of services
I disabled. It's a list for Windows XP, so I disabled most of the ones that were identical on Windows 7 and ignored the others. One of them broke my headphones. (and here's a better list with explanations of each service
, which might help troubleshoot which one affected headphone audio).
Even when broken, the Audio control panel could recognize when headphones were plugged in or removed, and changed state to reflect that. And it could play sounds on the headphone channel, as evidenced by the sound Tests actually causing the sound bar for the headphones to fluctuate. But it just didn't actually make it to the headphones.
Anyway, hopefully anyone else who comes across this looking for a solution might be able to investigate this as a services issue and find the solution. But best solution ime is, keep regularly updated full disk images that you can restore to.