It is only a bug with certain soundcards. I have a hercules game theatre xp 6.1 and have never experienced the problems. However, members of my squad with Sound Blaster cards are continually plagued with exactly the issue you describe.
The solution is listed in the previous reply. Just go to sound controls and click the checkbox for db boost. However, it seems a bit silly to me that a vox program should require a 20 decible boost just to make it audible...