I would suspect that Ubuntu is shipping by default with the sound muted, which is kind of nice some of the time; but I have no idea why my system should stay fixed while yours goes back to muted when you don't intend it to.I run Red Hat Enterprise Linux 5, that may be different from yours. But mine has something that may have an equivalent on your system.If I select the Red Hat at the lower left of the Panel (what Microsoft calls the Task Bar) and select Preferences->More Preferences and from that, select the menu item Sessions, I get a dialog box that has an item Automatically save changes to session with a checkbox in front of it. If you check it, all changes to your sessions will be saved. Otherwise they won't.Maybe yours is not checked. I have no idea what the default is, and it may be different on your system anyway. You might wish to find the equivalent on your system and check the box.