I used to use Mumble when I played WoW. The thing I loved about Mumble was that I could assign priority speaker to anybody, and when that person talked it would reduce the volume of everyone else who was currently talking. For Roll20, I would imagine a GM would be able to check a box in the game settings to always give himself priority speaker, or right click a user and select priority speaker from the menu there. Sometimes players can go off topic and the GM would like to proceed with the game. Simply allowing the GM's voice to be heard over everyone else's would help the game progress without having to ask everyone to quiet down. I know Roll20 doesn't handle its own voice chat service, but if this was somehow possible, it'd be an awesome addition.
