They really glossed over the sound, which is one of the more painful aspects of a Linux HTPC in my opinion. If you're planning on usual digital audio instead of stereo sound, be prepared for a rather nasty battle.
It really depends on the sound chip. My emu10k1-based SBLive 5.1 used to be really awesome for digital out (with a small hack to reset the chip every time I switched from DD/DTS back to PCM), but it has gotten worse over the past 8 years. I eventually switched to 5.1 analog output, sacrificing Pro Logic decoding for stereo sources.
Using OSSv4 might be a good option. Ever since I switched to it, sound has not been any problem whatsoever anymore.
I didn't find this at all. Plugged optical out on the motherboard into the optical in on the amp, set mythtv to let the amp decode dd/dts itself and everything worked fine.