In the episode Sideshow Bob's Last Gleaming, Sideshow Bob threatens to blow up Springfield if everyone doesn't rid the town of television. He appears on a giant screen at an airshow to deliver the message. Now, when the Colonel is talking to the airman, the airman says that he didn't want to take another order from that mans "snooty, high toned voice." When Bob delivers the message, he clearly still has his normal voice.