I once did Poseidon, posing as a fish monger, as Dustin Hoffman. "Whadda you know, anyway? Huh? Huh?"
Two of my best realized PCs of all time were very much "voice" characters. First, there was Brodacyn, an axe-wielding evoker specialist. He had a 6 Charisma, so I played him as an acidic, self-deprecating curmudgeon wearing wolf skins and encrusted with droppings from his falcon familiar. I used a slightly rasping, low, sulking voice, tinged with a Darth Vader like impatience. His response to "You'll know suffering as you have never endured before," was "I am already the most miserable person I know, so you'll have to do better than that." The Beast from Beauty and the Beast was also somewhat of an inspiration. He was a hateful, pessimistic creature who could foil attempt at diplomacy by his mere presence, and my group just loved him.
Another of my favorites was Cobin, a NG bard who served as the paladin's pragmatic-minded squire. His backstory was that he was a useless, poetry writing dandy, who one day, seemingly on a whim, became a devoted chronicler and squire of a grim fighter turned questing paladin. I used the most mincing, droll voice I could come up with, equal parts Rupert Everrett, John Cleese, and Cary Elwes. I appropriate the vocabulary of a WWI era British aristocrat. He was basically a twit, but if he drew unexpected ire, he had a polite, ingratiating patter that would do a prince's valet proud.