Remember that theory from long ago that red may be related to the playable character from Pokémon masters?

Well... if we discount that theory, there may be another reason why Red is "mute"

He just has selective mutism as a child.

Selective mutism is basically a kind of anxiety disorder that renders people unable to talk due to social anxiety. People with this disorder are still able to talk, but only if they feel comfortable enough with someone else.

This kind of fits Red, seeing that blue could understand what he says most of the time and the fact that he does speak once in his Pokemon masters EX (let's just call it PME) sync pair story.

Also, I did find a source that people with this disorder tend to be "intellectuals" which ties in with Red being a Pokémon champion. They also tend to have a higher EQ and understanding which is in a way evident from how Red is able to communicate with his charizard.

Still... I don't know how credible that source is so let's not take that too seriously.

So yea, that's all for this short theory...

Yea.