I may be 4 years late, but I believe Vader’s voice has changed from Star Wars to Empire to Jedi, due to much better production quality value, beginning with Empire, as each film we had experience a quality improvement on Vader’s voice, unlike in the first Star Wars film (not including post-2004 DVD release, though). By the time Return of the Jedi happened, Vader’s voice has been perfected in terms of fidelity, clarity, and authenticity due to the fact the third film sonically sounds superior than its two predecessors because of even far more superior production quality value and even far more superior recording equipment than Empire, and Vader has been sounding like that since Return of the Jedi. Same goes with C3PO’s voice as well.