All sound suddenly stopped about 6 weeks after I upgraded to Lucid. Everything seemed right but still no sound. On the volume control, I could even see the sound bar pulsing in sync with the music:

Volume Control

Another computer in the house has identical hardware and also runs Lucid but I couldn’t find any differences. Even swapped the sound cards as a test but still no luck. Here is what worked in my case:

1) sudo apt-get install gnome-alsamixer

2) Now open GNOME ALSA Mixer from Applications>Sound & Video>GNOME ALSA Mixer

3) Untick IEC958

Unticking IEC958

4) Enjoy music again!

UPDATE: To get the microphone working e.g. for Skype, need to enable Analog Source.