I think the OP is looking to change the volume of applications independently -- like selectively mute just Firefox or make iTunes (only) louder or make Adium softer. I'm not sure that Quicksilver can do that unless the app in question has triggerable volume controls (iTunes for instance, I think, does, but not every other application does)? Doesn't this trigger just mute the system volume? How is it any different than hitting the mute button on the keyboard?