Comment 7 for bug 337947

@Chris - Right click on the volume icon on your gnome panel and open the volume control dialog. Make sure the device is selected as NVIDIA HDA something (sorry I'm writing this from memory at work). Then click the Preferences button and enable the checkbox labelled 'Source' or 'Channel Options' or 'Channel Source' (sorry again I can't really remember what the option is called). This should then provide an option back in the volume control dialog to select either 2 or 6 channel. Also make sure you enable 'Line Out' and set the line out volume (since this controls the volume of the speakers). You should then get sound out of atleast the right speaker - the left one doesn't appear to work - see bug https://bugs.edge.launchpad.net/ubuntu/+source/alsa-driver/+bug/337314