Corrected entry: It seems that during the Luke/Vader duel in the carbon freeze chamber, immediately after Luke blasts Vader with steam, and again immediately after Luke kicks Vader down the stairs, you can hear David Prowse's voice through the mask instead of that of James Earl Jones.
christob
11th Jan 2010
Correction: That it "seems" to be someone else's voice is not a mistake, it's an assumption. Even if it were Prowse who made the grunts you're referring to(which hardly qualify as a voice), there are lots of things that go into recording and mixing sound. For instance, I highly doubt Vader's breathing is James Earl Jones's own breathing. This is no more a mistake than Vader's voice sounding like James Earl Jones when there's someone else in the costume.
JC Fernandez