two things about this:
first, wouldn't it be irrelevant since everyone will have it's own personal audio settings ingame? therefor standard settings should do best job, since you can "balance the sound" the way you need it....without messing it too much up for complete different settings.
and second, if i'm right, the overall volume is always "full", so as long all three sliders are on same positions, it doesn't matter if they are in the middle (default) or full right. afaik, with different settings of the three sliders, the most right slider defines the position "full" and the others will gradually decrease volume.
Don't know about the second thing, i'm not at my pc and can't verify it now. But for the first thing, use a neutral setting which you probably wouldn't use for playing. In this case, at least everyone who didn't changed the audio settings will have the experience you wanted to have.
sry for my english, but i guess you get the point