Comment 4 for bug 208139

When you do this:

1.) Open up a terminal & type "gnome-volume-control"
2.) Edit -> Preferences
3.) Check "capture"
4.) A "recording" tab should pop up. Click on that, slide the volume up if it's set to zero and click the microphone icon so that it doesn't have a red "x" through it
5.) Try RecordMyDesktop again. With any luck it should be working.

It starts without an error, but nonetheless, again no sound.

According to: http://ubuntuforums.org/showpost.php?p=3982775&postcount=4