We don't know many people who have this problem. You could try to update your soundcard driver.
Try also reducing the Sound-Accleration (I have no English Windows so the text is guessed):
Control Panel -> Sound and Audiodevices -> Advanced -> System performance
Move the Sound-Acceleration Slider to the Left (you can also try the second one (Convert-Quality).
Edit:
@olly
It looks like that some voices (Goblins, Treeman) are just filtered output of the normal voices. In one mission I had e.g. an Orc speaking with Goblin Voices and also Klaus one time .