Thanks for your input, I was afraid I'd have to try to figure this out myself!
I thought about this some more yesterday and did some troubleshooting, even though I don't fully understand what the cause of the problem is. My best guess is that when I alt+tabbed, the game activated the "mute when minimized" option, and then never de-activated it again. I hope this isn't it though, because I may not be able to fix that. I certainly can't fix it directly.
The details of what I tried is a little different than what you have written, because I'm playing on Linux. However, I uninstalled the game, deleted the account config (and backup) file, then re-installed the game. I think this should have the same impact as what you're looking for. I don't fully know what the installer does though (and therefore what to clean up), as there is no Windows registry etc. under Linux.
I went through my system logs, and I couldn't find any related error messages when I run the game. Everything seems to be okay, there's just no audio output to the device. I have 4 audio output devices. My primary speakers, my headphones, my monitor, and my television. As far as I can tell, the game is not outputting audio to any of them.
Is there any way to get some further information from the game, like a launch parameter to enable logging or something similar? It's quite frustrating that I'm getting no helpful output from the game at all, and I can't even select which device it should be using.
I'd be interested to hearing any other ideas that might help resolve this as well.
Thanks!