I 'accidentally' wrecked my gnome desktop (no menus, toolabrs etc.) - I had been doing a bit of tidying up and must have uninstalled something important. I could not decide what, so re-installed the whole ubuntu-desktop package.
This came with pulseaudio again, so sound stopped working like last time.
Decided to invest an hour in sorting it. No real success searching on the web, but I did find this article - http://ubuntuforums.org/showthread.php?t=962695. The author provided a little script to update the ALSA drivers automatically, and quite a few people reported that this cured the sound for them.
I am always wary about running unknown programs as root, but the script looked safe enough - just an automated way of downloading the ALSA source packages and running 'make install' etc. so I trusted it.
It ran and it installed ALSA 1.0.20, compared to version 1.0.18 in the Ubuntu package.
To my surprise, when I re-booted the computer, sound is back!