In order to get HDMI sound with these cards a fairly new alsa driver package is required. We used to ship an alsa-dkms package. However, since a while ubuntu ships allready compiled drivers of this version for each kernel update. 

So how you get your sound working now for these cards in yavdr-0.3 ?

You can easily install the driver with

sudo apt-get install linux-backports-modules-alsa-lucid-generic

For simplicities sake: reboot

After that you should see >with aplay -l< up to 4 devices like this:

card 1: NVidia [HDA NVidia], device 3: NVIDIA HDMI [NVIDIA HDMI]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 7: NVIDIA HDMI [NVIDIA HDMI]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 8: NVIDIA HDMI [NVIDIA HDMI]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 9: NVIDIA HDMI [NVIDIA HDMI]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Unmute them with alsamixer (you might need to change the card with F6). 

With speaker-test -Dplughw:"cardnumber","devicenumber" you can find the one which is actually working. 

Lets say this time, the following was successfully:

speaker-test -Dplughw:1,7

Then you can set them manually (note if there is only one device, it will be automatically detected).

dbset system.hardware.alsa.hdmi.device=7
dbset system.hardware.alsa.hdmi.card=1

Now go to the configuration web interface ("System" -> "Sound"), make your choice ("HDMI Stereo" most likely for TV sets) and you are done. 

If you want to do it "right" or you have the problem that the order of different sound cards changes, please read this page:
http://wiki.xbmc.org/?title=HOW-TO_set_ ... C_or_GT240

Try some of them until you only have one device shown in aplay -l