I'm not sure Chris knew much English in this movie. I've heard that "Highlander" was his first fully English speaking role, b/c me only spoke French before that. I know that when the directors / PTB for "Highlander" met Chris for the first time, they didn't realize he only spoke French, so he had to learn English for their film. I figure "Greystoke" was mostly him 'mimicking' words.
But yes, he does have a quiet voice. At least he doesn't mutter the whole time like Heath Ledger in "Brokeback Mountain" - ugh that was annoying.
I'd also heard that in "Greystoke" they dubbed Glen Close's voice over Andie MacDowell's... which is kind of odd.