We as parents don't always have to have an answer. If you pick an answer, it might make sense to you, but make more questions from the child because it doesn't make sense to them.
What I did with my kids was give the question back to the child.
"I hadn't thought about it. Why do you think they don't talk?"
They came up with some good possible explanations, including
-saving their voices for the show
-they can talk in movies, but not out in the world
-they can't speak every language, so to be fair to all the kids, they don't speak when you meet them
-during the shows they wear special translators so people can hear them
That way, I was not bringing up anything that would cause problems later and cause the child to ask more questions (like, "I could hear Mickey in the show, so why couldn't I hear him later? I still believe?")
It's also important to realize that other kids may have been told different stories by their parents and believe what their parent told them. I heard/saw some kids playing together while waiting for the parade and the question of why the characters don't talk came up.
It ended up being a "MY mom said this is why .........."
"Nah-uh. That's not right. MY MOM said this is why .............."
It ended up being quite a long (and not very happy) discussion, with both sets of kids saying to their parents "Why did you tell me ........ and his mom told him >>>>>>?"