Here's why:
* MAD was known for parodying popular culture: The show often used iconic voices and characters for comedic effect, and Mickey Mouse was a perfect target for parody.
* "Soul Tron" was a play on "Soul Train": The name itself is a clear reference to the popular dance show. MAD frequently used wordplay and puns, so it's logical they would use Mickey Mouse's voice to poke fun at Soul Train.
However, without specific details from a particular MAD episode, it's impossible to say for certain.