Solved!
I loosely followed a posting at:
http://ptspts.blogspot.com/2009/08/asrock-ion-330-nettop-with-
jaunty.html
and created an /etc/asound.conf file with contents:
defaults.pcm.!card NVidia
defaults.ctl.!card NVidia
defaults.pcm.!device 3
defaults.ctl.!device 3
Now HDMI sound just works.
I have the same problem as rafael.inigo : even tho I have unmuted iec958
in alsamixer, there is no sound by default. Oddly enough tho, sound
over hdmi seems to be working under the hood, since if I issue something
I found in a Googled posting:
mplayer -ao alsa:device=hw=0.3 -afm hwac3 test.mp3
