About Ventrilo:
If you are using a Hercules or Philips sound card then this is most likely the problem. Usually associated with any sound card that uses the Crystal CS4630 chipset.
This is a well known problem and is systemic to these brands of boards / chipset. However, these companies do not seem to forthcoming with a solution. Your options are:
1) Dump these cards and get a SB Live/Audigy or a TB Santa Cruz.
2) Try using a USB based microphone.
3) Enable any onboard sound and use it for your microphone input.
4) In setup enable the "Use DirectSound for input device". This might help.
Hercules owners should also consider installing the latest 4101 drivers. These drivers were reported on the support forum to have fixed one persons sound problems. Guess it can't hurt to try.