Thanks to obedlink (http://ubuntuforums.org/showthread.php?t=2028550)

If you have problems with the center and Sub-woofer then try this solution, in the console type:
alsamixer and with the arrow keys, Step onto Center / L and press m, what happens is that the Sub-woofer and center speaker are muted.

Center/Subwoofer Muted

Center/Subwoofer On