I recently introduced a handful of NPCs in my long-running D&D game that have goofy voices. Since I routinely juggle a lot of characters, some of which are only played occasionally but have distinctive voices, I should probably record my notes somewhere. (Were I really slick, I ought to actually record the voices, but that's only useful if I go back and listen to the recordings.)
Goruthrel: The wizard's new familiar, a spider summoned from Faerie. Vaguely midwestern — not quite a Minnesota accent, but that same sort of wholesome, overstimulated, swear-less mode of speech. She always seems scattered and disorganized. Put a lot of extra stutters and "ums" in your speech, and make sure to click your tongue and say, "Whew," when appropriate. Despite not swearing or even being terribly vicious, she is from Faerie, so occasionally say something totally untoward. ("Oh, well, I mean, ah, I guess we could just, just go right ahead and kill them all, right?") To get into character, my insertion phrase is, "Oh, oh geeze. Oh my." She always sounds flustered, but is actually pretty smart. (This is in direct contrast with the wizard's previous familiar, a quasit who was gleefully waiting for him to die.)
Tetposmeton: A devil and loan shark, cutting deals in Sigil. (An Amnizu, if that matters.) Smile incredibly wide while talking; if it almost hurts, you're probably smiling enough. He talks like some manner of mutant, Frank Zappa-esque radio announcer or disc jockey (the phrase "ice pick in the forehead" from "Fembot in a Wet T-Shirt" is a good insertion phrase), like Casey Kasem on entirely too much cocaine. If someone seems iffy, keep talking — bombard them with so much information that they'll eventually agree to the whole thing. You're somewhere between a used car salesman and their best friend; all participants know you're corrupt, so you might as well go whole hog.
Tingletam Finglefam: A creepy gnome alchemist. As per Noel Fielding, he's from Souuuf Londen. Sounds like The Hitcher when he's being gross, not when he sounds like an old man. Talks with his hands in this demented, Rumpelstiltskin-like fashion, like some sort of marionette. When I need to get into that character, his key phrase is, "stony giblets." Say, "stony giblets," in the Cockney accent, and I'm ready to go. Just be vaguely unseemly and gross.