Changing the voice to match the prequels was a bad idea. Given Fett's line of work, you don't want your real voice coming out of that helmet. You'd never be able to relax or go incognito in public.
In what little of the EU I've read, Fett has snuck around out of suit to catch his prey. It's not all flying around with guns blazing.
Why wear a mask at all, if everybody knows you're Jango's clone kid?