Rebooted and tested playback again. This time I got an MC playback error about not being able to play WASAPI 44.1 kHz 2 channel. I switched it to Direct Sound default output with same error. Opened Realtek HD Audio Manager to see what I could see and found Dolby Pro Logic IIx turned on. I haven't touched my audio settings in ages, so I couldn't remember if that had been on previously or not, but, in the spirit of trying everything, I turned it off and switched MC back to WASAPI. Lo and behold, I have sound once again.
Dunno how or why, but thanks to everyone trying to be helpful.