Since Frank Welker was playing Shockwave in the movie, I kept thinking what Shockwave sounded like in movie. Tried to compare other Frank Welker cartoon characters. Until I stubbled on to a clip of the beginning with Shockwave saying "Optimus". That made me think is that Frank Welker's Megatron voice. Think about it, Shockwave when he said that one line, he sounded a little deep and wheezy like Megatron's voice, with just a few beastly roars and growls. And since Frank Welker didn't get the part for Megatron, I guess you can't let a good Megatron voice go to waste.