The leader of the Autobots grants an interview.
QUESTION: Where did the Optimus voice come from?
CULLLEN: I was actually impersonating my brother Larry. He had served in the Marine Corps. in Vietnam. A Purple Heart recipient, as well as the Bronze Star. And when he came home, there had been a significant change in him. His voice was the first thing that I noticed and so I quickly began impersonating that. And he would says things like [in perfect Optimus Prime], I think you're going a little far with that, Peter!
I can see and hear that.