Try logging into command line as root and typing:
This will probe for hardware and what not. When done, type this:
This will get you into the configuration menu for alsa. Type M
to mute/un-mute any chanel. Use your arrow keys to adjust volume settings and to move from channel to channel. I dont' have 5.1 or surround attached to my box, but there is a Surround channel in there. When you're done, hit Esc and you'll get back to command line. Now type:
to save your settings.