After all, you couldn’t have a squeaky high pitched Darth Vader. James Earl Jones voice is so deep and so manly, that it does add considerable drama to Vader’s presence. Which is why it was chosen for the Dark Lord by George Lucas. read more
Voice synthesiser. The books sometimes call Vader's voice "a deep, synthesised voice" (I think Dark Lord: The Rise of Darth Vader does). The Visual Dictionaries for Star Wars might even show which bit of the helmet's doing it- they usually show various things in high detail- like the inside of a stormtrooper helmet. read more