[malibu327]

Running the speaker test helped me find it.

Your explanation makes sense, Plaguester. But in my case, it is going from the xbmc computer to a Denon receiver (via HDMI) which supports the various audio codecs and then from there to the TV.

Good. Run through these (some of them you've probably already done, but I prefer to give the list in a complete fashion for future googlers):

1. Temporarily remove any changes that you made to /etc/asound.conf or ~/.asoundrc by renaming the file to something else.
   
2. If "aplay -l" indicates that your NVidia HDMI device is on card 1, rather than card 0, disable the onboard sound in the BIOS.
   
3. Unmute all "S/PDIF", PCM, "IEC958", or digital entries in alsamixer ("MM" is muted, "00" is on; 'm' toggles it).
   
4. If you disabled the onboard sound in step 2, run "aplay -l" again and see if it looks different than before.
   
5. Run "aplay -L" (yes, upper case L) and see if you have a clear listing for an HDMI out ("hdmi:blahblah"). Run "speaker-test -D plug:hdmi" (Ctrl-C quits). If you hear sound, skip the next step.
   
6. Run "speaker-test -D plughw:0,X" where X is the device number until you hear noise (Ctrl-C quits). Note the device number that worked.
   
7. In XBMC > Settings > System > Audio Output, set "Audio Output Type" to "HDMI"; "Speaker Configuration" to your setup; "DTS Capable Receiver" to "On" if and only if your receiver can handle it; Ditto for Dolby Digital.
   
8. For "Audio Output Device" and "Passthrough Audio Device", start with the simplest device (probably something like "HDMI (ALSA)"). Play a file with DTS or AC3 audio. If that doesn't work, try the one that looks like "NVidia - HDMI - X" (where X is your device number). If that doesn't work, try "custom" and type in "plughw:0,X". Some hardware could also be reported as "IEC958". My receiver reports the switch to "Dolby Digital" as soon as the file begins playing.