I was having trouble with vox after setting up a new computer.
I had the same problem before, that is, its ok for a while then it just packs up needing a reboot to get it working again.
I was trying to remember what the heck I did to get it going again when I remembered dxapplet!!
With dx8.1 the sound debug level is set a couple of notches up. Dxapplet gives you a icon in control panel to access the debug level. After installing it again & setting debug to zero voice now works ok.
Just to remind people who may, like me have reinstalled etc, to try it if haveing similar problems
Get it here