From wikipedia:

Peter Cullen, born on December 1, 1944 in Montreal, Quebec,[2] is a Canadian voice actor who is best known as the voice of Optimus Prime and Ironhide, in the original Transformers series and the narrator in both American Voltron series (he would subsequently reprise his role as Optimus in Michael Bay’s live-action Transformers film series). In addition, he has played Coran and King Alfor in the Lion Voltron series, the transforming spaceship/robot Ramrod in the 1980s anime series Saber Rider and the Star Sheriffs, Commander James Hawkins in the Vehicle Voltron series, Eeyore in Winnie-the-Pooh, KARR in Knight Rider, Antor and Bomba, and Gunner, in Dino-Riders, and Mantus in The Pirates of Dark Water. In 1968, he appeared as ‘Giles’ with Joan Stuart as ‘Penelope’ in ‘L’Anglaise’, a CBC comedy about a French-Canadian husband with an English-Canadian wife (CBC’s ‘Funny You Should Say That’).

Watch this video and be amazed at how they did the voices in the first Transformers film. I know that Revenge of the Fallen is out, but hey, Transformers it still Transformers!