Typically, I would look at the sound card and see if it is sharing an interrupt with another medium to high usage device. I am talking about the physical interrupt, not the virtual one.
If the sound card is onboard, then I can see losing VOX being something which will just happen. Some of the newer onboard sound chips seem worse. The Realtek HD audio chips seem to be more problematic than the previous Realtek chips.
Also look for other applications or utilites which are using sound. If thy interrupt the games sounds, then that might cause it as well.
