I don't think the fix you mention will work.
http://forum.ppcgeeks.com/showthread.php?t=11799
That effects volume in general with voice command. The Volume on the handset it plenty loud and is specific to the headset only. It might be controllable in the \Windows\AudioPara.csv but I have not taken the time yet to bother experimenting because it doesn't really gain you that much functionality.