Like many others, my speakers stopped working after I upgraded to Lucid Lynx. The usual solution of purging, reinstalling PulseAudio didn’t work for me, nor was my volume muted.
I finally got it working by removing PulseAudio and not installing it afterwards. AlsaMixer works just fine for me. However, I some times get the feeling that the sound quality might have deteriorated a bit, but it might just be me worrying too much.
This post is for those with a similar problem. If the usual solutions don’t work for you, try removing PulseAudio. That just may be it.