Ok I think I might have fixed it. Whether I did it the "right" way or not I don't know, and don't particularly care.
So just in case anyone else runs across this, here's what I did.
Noticed in Services my Window Audio wasn't running despite being set to Automatic, started it, it started fine. Rebooted, and verified it started on it's own this time. Audio still isn't working.
In Device Manager, uninstalled both instances of the High Definition Audio Bus, under the assumption that I'd SHUTDOWN (not restart) and then start the PC and it would auto detect them and reset them up. In reality, by the time I was done uninstalling the second one, Windows had already reinstalled the first one and it didn't have the yellow triangle indicating a problem anymore.
I shutdown and restarted anyway, and when it came back up, everything was working, audio is back, both HD Audio Buses appear to be working correctly, and Windows Audio restarted and stayed running on it's own.
Which of those steps was the "magic" one, I'm not sure.