Orcish: Blah blah blah, grah, hurrah! Zug zug and daboo, loduk, eekdork and yodai. Rahaha.
Kobold: Yip yip kiyyip. No take candle.
Gnoll: Arrrrr rarf! Jibber jabber.
Dwarves: German and/or Russian.
Common: French. :O Or possibly latin. Could also be english.
Half-elf: Spanish and/or french. Yep, it's an excuse to include the romance culture.
Elf: Stupid sing songy language.
Auran: Um. Humans can't pronounce this language. It's why wind likes to howl sometimes or whatever.
Draconic: Latin again, or possibly greek since that's even more eldritch, somehow.
Infernal: Dunno what reverse latin would sound like, but you could make it sound druidic too, if you like. Or whatever sounds evil. Language that sounds like something out of Cthulu mythos works too.
Terran: American south.
Goblin: Sounds exactly like orcish, because all those creatures pretty much fulfill the same purpose as humanoids. Somehow though, it's not orcish. Sorta like how the aliens in that first halloween special of the simpsons speak a language that sounds like english, entirely by coincidence.
Abyssal: Slipknot magically starts playing whenever they speak. Gets annoying after awhile.
Aquan: The most exotic language of them all! You have to fill your lungs with water to understand it, meaning you have to be dead. Needless to say, it's also the least interesting language.
Celestial: Latin again. Why? Because gregorian chant is in latin. Also, angels have sing songy voices, because they're angels.
Ignan: Sharp and angry spiting. It's basically any language you like, but you have to accent it with ANGER, otherwise no one will understand that you are speaking in Ignan. It's a weird culture.