Fresh install of 11.10: Right after installing restricted drivers (nvidia) all audio devices disappear. Why could it be?
EDIT: Also, after disabling these restricted drivers, the audio devices (sound card, webcam) reappear.
A way I found to temporarly fix it was to kill the "pulseaudio" process and wait for it to begin and to load anew. Just in case somebody else has the issue.
Modern Nvidia cards that have HDMI outputs have the option of acting as a sound card (so you can get audio over HDMI). What I suspect has happened is something has detected this new sound card and has changed your audio settings to HDMI.
Open up the Sound Preferences screen and you should be able to configure it something like this: