I've been wrestling with this problem for two days. Since upgrading from 10.10 to 11.04, I can no longer get multichannel (5.1) sound to my speakers. Stereo 2-channel sound was working, but it was using the onboard sound system. I want 5.1 surround sound using my Soundblaster Audigy2 card.
I have now disabled the onboard sound in my PC's BIOS. Now, ALSA is using the Audigy card. Pulse Audio is using the Audigy 5.1 as both source and sink. This is verified by monitoring them in padevchooser when a sound is being played. However, no sound is going to the speakers, which are wired into the three output jacks of the Audigy2 card.
All of this worked just fine on Ubuntu 10.10.