It has to do with tracking the levels in the group. You want to be able to slide a master level up and down and have the levels maintain the "same" balance, ev

It has to do with tracking the levels in the group. You want to be able to slide a master level up and down and have the levels maintain the "same" balance, even if one of the players ends up going to (effectively) zero level when you have turned it down. Takes a little bit of thinking about how you represent the range of the levels. I think the patent is actually quite a bit broader, and covers using a UI that has a master volume control across players whose individual volume levels are displayed as well. Google pulled all "master" level control in response.