From  When I compiled the kernel driver for my CM8738 audio card and start playing on the device I was able to hear only 4 of my 5.1 speakers - the front and the rear speakers. I have tried all of the alsamixer options and the only thing that I was able to achieve was to turn off the rear speakers by disabling 4-channel mode.
Now I am still looking for a way to listen 5.1 Surround sound with that card.
I had this problem. If you enable virtual 5.1 surround sound it works (found in the audio rack settings). Also for me the centre and sub channels were the wrong way around so I had to cut up the audio cable and swap them