I've always really struggled with different OOTS characters and giving them consistent voices in my head, so I've recently decided (on my second readthrough of the whole thing) to start trying to give their voices some more character. Here are just a few that I can think of immediately.
Roy and family - Midwestern accent.
Haley and Ian - Thick Kiwi accent (if you've watched Castle in the Sky, think Sheeta all grown up and you've got Haley).
Elan, Nale, and Tarquin - West Country accent (If you've played Portal 2, think Wheatley and that's basically the voice I give Elan. Tarquin would sound more like Hagrid with his voice lowered about half an octave).
Durkon - Scottish (Duh)
Belkar - Duke Nukem if he grew up in the Shire.
Daimyo Kabuto - George Takei. That is all.
Miko Miyazaki - A really raw hoarse voice, like someone who spent too much time screaming when they were young, and still spends too much time screaming.
Xykon - Raspy Ashkenazi New Yorker. (idk)
Redcloak - Crisp Oxford Accent. Think JARVIS from Iron Man, and you're spot on.
Hobgoblins - Football Hooligans, the lot of them. Basically, 40k Orks.
Oracle - Billy Crystal
Thog and Orcs - Russian accent, very deep voice.
I'm trying to keep accents roughly the same for characters of the same place of origin (I guess Greysky City is New Zealand?). What about the rest of you? What sort of voices do you imagine for the characters?
I have specific voices that I imagine (and frequently vocalize) for each (major) character, but the easiest one to describe is Belkar, who sounds like Cartman but even more pissed off than usual.
Because seriously, just try saying "I am gonna rip off your stupid bug head and piss down your neck hole!" without sounding like Cartman. It doesn't work!
Notably, none of my voices have any accent as such at all, not even Durkon, beyond a "amateur actor struggling to approximate the sounds represented in the text" sort of way. I therefore find your random assortment of accents from all over the world quite interesting as well as inexplicable.
Edit: Ashkezani New Yorker? That's not a villain accent it's a comedy accent! What kinda villain you ever heard of talks like a overexcited Mel Brooks? I'll tell you what kinda villain, it's a Mel Brooks villain played by Mel Brooks, that's what. And that's what makes it a comedy villain! (for full effect, make sure to read the above as if recited by Mel Brooks)
My Xykon's voice, meanwhile, is vaguely similar to Samuel L. Jackson when he's trying to act friendly, maybe with some extra bass added in post.
My Redcloak's voice (and by extension the voice of most other goblins and hobgoblins, depending on their role and mood and so forth) is fairly high-pitched and very nasal, sort of like either Dinobot or Tarantulas from Beast Wars only without the extra-breathy quality.
My Thog talks like Yogi Bear, or possibly Bizarro from the Superman animated series, if they were really happy all the time. I don't think Thog could manage a Russian accent even if he was speaking Russian. An accent like that requires a much higher int score than verb conjugation.
edit again: My MITD voice sounds like my old friend Jerry, which is kinda like Cartman only if he was half-asleep and smoked a lot of weed, and ended every sentence with a question mark.
Where's what's-her-name, the chick with the pigtails?
Jirix: Tsukiko? She was here when the sewer team reported in...
Anyone seen her since?
Demon Roaches: No. Nope.
Not since I brutally murdered her ten minutes ago, no.