I recently had the problem to enable 5.1 Sound with 3 click outputs on an Nvidia ION board. By default Linux configures it with only one plug configured as output. After some research I found a solution in the OpenSUSE Wiki you have to set during module loading to get it working.
Basically you have to add
options snd-hda-intel <strong>model=3stack-6ch</strong> enable=1 index=0
to your modeprobe.conf to tell the driver to use all its 3 plugs as output. After you loaded the modules like this, alsamixer will show you all the 6 channels, so you can unmute them.