Interesting points being brought up. I don't wish to dispute them all; some I agree with, some I do not. However, it is important to note that his breathing and speaking are not tied together. Most of his breathing is machine assisted, possibly something like a tracheotomy. Hearing Sebastian Shaw's lines at the end of ROTJ reveal that he clearly has much difficulty getting sufficient air through his own vocal chords to do much, and his mask takes what little he can produce and turns it into his James Earl Jones voice. This distinction between his speaking and breathing is not just apparent in this film, but in all others featuring Vader in-suit, as well as in the novelization of Episode III. This is why Luke was so concerned about taking off his mask: his mask is not merely an air filter, but rather an actual pump.