Sound volume for applications can become locked below global output volume.

Normal behavior, as far as I understand, is for all applications to have the same sound volume unless you lower the volume for one manually in the applications section of the sound settings. However, some applications have their own own volume control that only affect the volume of themselves and the global output volume, not other applications. This can put those other applications below the global volume where they become stuck.

An example: you have the output volume at 50%. Rhythmbox is locked to output volume and is also at 50%. Then you open a movie in totem. Sound volume is low for that movie so you use totem's volume slider to increase volume to 100%. When done you close totem and use the media keys on the keyboard to lower the volume back to 50%. Since rhythmbox stayed at 50% instead of following to 100%, it got stuck at half of global volume and after lowering is now down to 25%. It can never go above 50% with just the normal volume controls, you need to go into the applications section of the sound menu and move its volume slider to reset it to the same value as the global output volume.

This is especially a problem since you might increase the global volume a lot in order to compensate for low volume, only for a newly opened application to start playing sound at way too high a volume, a danger to your ears and possibly your sound hardware.

The same thing happens with the volume control in vlc so it's not just totem. I don't know if it's a bug or not, but it sure is confusing and annoying. I was about to bug report it, but I don't know which package it would be.