I recently tried to connect a Samsung led- tv to a simple Nvidia G210 display adapter, causing my analog sound to dissappear completely.
The fix can be found as decribed here:
First update nvidia drivers to the latest:
sudo apt-add-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update sudo
apt-get install nvidia-current
Then following this instruction found on the mythtv website:
In short, the /etc/X11/xorg.conf needs to be adjusted with a modified EDID which prevents audio to be send through HDMI:
Open your ‘/etc/X11/xorg.conf’ file, and go to the Device section for your video card. Add the line
Option "CustomEDID" "DFP-0:/etc/X11/edid.modified.bin"
to tell the driver to use your custom block rather than acquiring one from your TV. The changes will take effect after a restart of X.