Well considering that this is Tatooine and a lot of parts are scavenged, one can assume droids are cobbled together from disparate parts - old vocoders from widely different droids etc. That's part of the charm of Tatooine. It's gritty.
Also we've seen 3 protocol droids in the saga, all of which have different voices, so I don't see it as a problem. Try not to think of 3PO when making a judgment on the voice. I just want to know if it works as the voice of "A droid".