There has been a completely new development in this saga. One thing I had also been investigating was to see if I could somehow get surround sound through the onboard Intel sound chip. This chip does not have a iec958 (optical) device however, so it wasn't going to work directly with my receiver. As a temporary workaround I was attempting to at least get stereo sound going through the line out of this chip, and it was not working, despite the fact that it worked fine on another machine in the house with an identical motherboard. Then I remembered that the UEFI has settings for the onboard devices, and by default the onboard audio is set to "Auto", which it turns out means Enabled unless there is a sound card installed. So that's why it didn't work on the machine with the Xonar card installed. I changed it to Enabled and I got stereo sound. I can also now see four separate HDMI sound devices in aplay. So I played around with some of the things I have learned recently, and it turns out that this chip can do surround sound via HDMI, and since the TV that the system is hooked up to can pass that on to the receiver, I decided to try it, and it mostly works. I say "mostly" because when I run the 5.1 surround sound test in MythTV, it does front left, center, front right just fine, but "surround right" means both rear speakers at reduced volume, and "surround left" means both rear speakers at normal volume. That's of course not how surround sound is supposed to work, so I still have a few things to look into there (including things like settings on the TV).

So I'm still interested in trying to get the iec958 device on the Xonar card to work, because I expect I'd get better quality sound that way, but the urgency is reduced.

--Greg