Check out the following posts, they will surely help you!
- HOWTO: Ubuntu 10.10 Nvidia hdmi audio
- Re: ion2 hdmi audio alsa/snd_hda_intel doesn’t think my tv is connected
For those who also use a Zotac GT 430 (under Ubuntu) ;
- The following channel was a succes :
aplay -Dplughw:1,9 /usr/share/sounds/alsa/Front_Center.wav
- Added “load-module module-alsa-sink device=plughw:1,9” to /etc/pulseaudio/default.pa
- Changed “/usr/share/alsa/alsa.conf
defaults.ctl.card NVidia
defaults.pcm.card NVidia
defaults.pcm.device 9
Karim Vaes 