He's a good enough voice man that you don't notice it automatically, but... oh, jeez... once you know, you can HEAR it. And you start hearing Prime speaking in Eeyore's voice, and vice versa.
"It's snowing still," said Prime gloomily.
"So it is."
"Yes," said Prime. "However," he said, brightening up a little, "we haven't had a Decepticon attack lately."