I've had this same problem in Ubuntu 10.10 and earlier and have never found a solution. I forgot how to change pulse audio and look at the MXSA because that was awhile ago and I haven't messed with Linux since.
What have I done to try to fix this? I turned the sound up lol. It was on mute after install.
One thing I would like to mention that I have never asked about before is, I do not have a sound card, I used on-board audio from my motherboard. Could this be a problem?
I have no idea what to do to fix this, as I am not a expereinced Linux user and any help is apprecaited.