I had the same issue and the alsamixer trick did it for me.
Two things to note:
1) The instruction I was following said to exit X Windows before running the mixer (ie don't just open a terminal)
2) when I ran it, the channels to un-mute were off the screen, had to cursor right to see them.
The other much more simple solution is to use OpenElec instead.
It is a stripped down linux distro that boots strait to XBMC
It installs to and runs off a USB key and it worked out of the box on my Asrock woth zero fiddling needed (and has lightning fast boot time)
With a bit of fiddling you can get it to dual boot from the HDD with you other preferred linux distro, I couldn't be arsed myself, I just plug in the USB stick when I want XBMC.
Note that I use the SPDIF socket strait from my Asrock for surround sound. You can run into a lot of problems getting 5.1 to go HDMI to your TV then SPDIF to your surround. Most modern TV's will down mix it to stereo.
You really don't want to resort to MS Windows