The Yak is voiced by Billy West. Why he wasn't able to reprise his roles of Ren and Stimpy is a mystery.note And considering John Kricfalusi didn't have a hand in the development of this game, it couldn't necessarily be West's Creator Backlash towards John K. as detailed in the Ren and Stimpy trivia page, as he did reprise the role of Stimpy in the equally John K.-less Nicktoons: Attack of the Toybots.
Six cases for playable characters. Ren and Stimpy went from Billy West to Chris Edgerly and Eric Bauza respectively (Though Bauza was Stimpy's voice in Adult Party Cartoon), Powdered Toast Man went from Gary Owens to Corey Burton, Danny went from David Kaufman to Keith Ferguson, Dudley went from Jerry Trainor to Jack DeSena, and Aang is not Zach Tyler Eisen, but Benjamin Helms instead.
Oddly, despite not being able to voice Ren and Stimpy, Billy West does provide voice acting in the game (for Yak).
And to a lesser extent, commentator Perch Perkins is voiced by Matt Yang King instead of Dee Bradley Baker.