This morning when logging in to my newly-upgraded-to-Ubuntu-10.10 laptop, sound was not working. It appears that the solution was very simple: my user was not authorised to use audio devices. I don't know why it was disabled as it had always worked fine before but it's very easy to solve so if you have the same problem check that first. To resolve it, go to System → Administration → Users and Groups, select your user, click on the Advanced Settings button, enter your password, click the User Privileges tab and make sure the Use audio devices box is checked. While you're at it, do the same for the other users on your system.
If that doesn't work, there is a handy wiki page on debugging sound problems.