Links: nV News thread
, the XBMC thread
, and the XBMC Wiki page
. So go ahead and read through those if you haven't, and take it to one of the open threads if you're still stumped. Yes, I know the nV News thread isn't titled with your card, but the HDMI chip is the same, and it's easier to keep all that together.
All that said, the device setup in ALSA seems to be the problem. One of your speaker-tests used the nonexistent-ever device "hwplug" (should be plughw); besides that, your ALSA configuration files are probably messed up.
If you don't need any per-user configuration of ALSA--and for now, you don't--delete all /home/*/.asoundrc files and use the /etc/asound.conf listed at the bottom of the wiki page. If you're still having problems, please repost to the main thread. Thanks!