Here's Walt Disney as the Mystery Guest on the 11/11/56 episode of What's My Line? Years ago, I met an animation art dealer who had acquired, at great expense and trouble, a 16mm print of this appearance. He was in the business of authenticating Walt's autograph and this was the one example he had of a Disney signature that he was absolutely certain was Walt's. As you might imagine, there are a lot of forgeries around…including many that came from Mr. Disney's office but were signed by someone else.
Note if you will that Walt says something about not wanting to disguise his voice with a falsetto because that would be "The Mouse." I'm not sure if he was afraid of being too recognizable if he did a voice in Mickey's range or if he just didn't want to be seen on TV with that voice. With one or two minor exceptions, he always avoided doing Mickey in front of audiences or cameras. The Disney organization was thrilled to have Clarence Nash demonstrating how he spoke for The Duck. They even kept him on the payroll to do that. But Walt seemed to think it was undignified for him to be seen talking like Mickey Mouse and there's very little footage of him speaking in that register.
Also: By 1956, he'd pretty much given up doing Mickey's voice, passing the mantle on to Jimmy MacDonald and occasional others. One of the last times he spoke for "The Mouse" was in the wraparounds that were animated for The Mickey Mouse Club in 1955. The little films where Mickey welcomed viewers or told them to make sure to watch tomorrow were Walt's last real performances as his studio's superstar.
Here he is trying to stump a panel that included Mr. Jerry Lewis…