I guess I don't know what you're talking about. I'm clearly not using the A40 and I can set voice volume to any level I want. I can have 100% game volume 0% voice volume, 100% voice volume 0% game volume, or absolutely any mix of the two that I want, utilizing the voice/game volume knob. My headphones/mic go into a 3.5mm male to two 3.5mm female splitter that I picked up from Turtle Beach (it came with the HPX I bought at the time). If I were plugging only the headphones into the mixamp, then yes... There might not be any volume mixer (haven't tried it). As it is, using the 3.5mm splitter (which is dirt cheap online) I can and do control the voice/game mix as intended by Astro. I do so every night playing games on a Xbox360.
@Jae304: I'll probably clean up the tape job at some point. It took me all of 5 minutes to mod a boom mic for my headphones... I suppose I could spend another 5 and make it more presentable. What I really want to do is flip the mic so both mic and headphone cable are going down the same side, but it's not that bad actually.