Zhijn: Try this topic (ignore the solved mark on my name zhijn!).
Maybe you can find yourself a solution:
http://www.gog.com/en/forum/the_witcher_2/voices_have_disappeared There was non for me tho. =(
I've played through the game twice without incident so I'm not sure that its the same problem :/
And I don't use G35 either.
Oddly enough my save is right after that sneak mission, however a new game still has no conversation :<