I'm having similar trouble. I notice it on YouTube, and I think occasionally on Rhythmbox where the sound will not go down. I can't get it down using the buttons on my Dell Inspiron 1525 or through the icon the system tray. Sometimes even muting it will just make it go to the highest point.

I'm using Gutsy.

The sound mechanisms freeze, and then a little while later, they are okay.

Tracy