I don't know if this is applicable but for my HDMI and My Brothers to work with an Nvidia card we had to do this weird configuration to tell Ubuntu where to find the device. Here is my notes; I hope it helps.
You will notice that aplay -l, shows 4 sub devices, the first thing you need to ensure is that none of the 4 devices are muted, you can do this using alsamixer (where the number after c is the card number of the NVidia audio shown by aplay -l):
$ alsamixer -c1
Now you just need to unmute the S/PDIF devices for HDA NVidia in alsamixer by pressing m for any that show MM so they end up as 00, figure out which one works, and add a line to a file. Try the following and substitute 1,3 with 1,7 1,8 or 1,9 until you find which works:
aplay -D plughw:1,3 /usr/share/sounds/alsa/Front_Center.wav
Mine is 1,9 so I added a line to /etc/pulse/default.pa after alsa-sink line:
load-module module-alsa-sink device=hw:1,9
Reboot. The only odd thing it that in the Output tab of Sound Preferences you will need to select the HDA NVidia device that does NOT say HDMI to get HDMI sound. The speaker test in Sound Preferences will not work, but if you play Rythmbox, etc. you should be able to switch sound output on the fly.