JK Simmons as Soldier
David Sobolov as Heavy and Demoman
Jeff Bennett as Engineer
Wally Wingert as Medic
Diedrich Bader as Sniper
Maurice LaMarche as Spy
Tara Strong as Miss Pauling
Corey Burton as Merasmus
Tress MacNeille as the Announcer
Charlie Adler as Grey, Redmond, and Blutarch Mann
Daran Norris as Saxton Hale
Honestly, anybody could be Pyro, he's a non-speaking character that makes random grunts and gibberish noises. As for Scout, he can be played by any "young-sounding" actor.