For which version? both?
the DOS version runs well under DOSBox, the Windows version runs well under ScummVM. As such, either version has different settings to modify.
EDIT: Ohh... wait, you mean the music plays but the voices aren't heard? All the voices
or just the narrator's voice? If I recall correctly, the DOS version of KQ6 has written narrator lines that are not
spoken during the opening movie, though Alexander, Valanice and Cassima do have speaking parts. It's the Windows version of KQ6 where the narrator's lines are spoken during the opening movie.
DOS intro: (no narrator voice) http://www.youtube.com/watch?v=Z_lYIBSwot4&t=50s
Windows intro: (voiced narrator) http://www.youtube.com/watch?v=YTBKp9vxlT4