>>42929346A lot of depictions of Red as an NPC show him being understood by others, even though he doesn't appear to say anything. Notably, Masters shows Blue relaying what he's saying to you.
Is he speaking too softly to hear? It doesn't seem likely. He's talking directly to your face, and, when playing as him, people can understand his yes/no answers.
Obviously, what's happening is a little bit of a narrative joke. It's the principle of a silent protagonist, but applied to an NPC instead. However... Red isn't special in being a silent protagonist. Every Pokémon game has one!
But while the other protagonists went on to speak normally after their times as the player character were over, Red got stuck.
He can't speak—not because of a physical or mental disability, but because of a temporal anomaly. The universe still thinks he's still a player character.
Upset by this, he secluded himself from society and trained on Mt. Silver for three years; up until a boy from Johto seeked him out and beat him at his own game.
Since then, he's made up with Blue and started to reluctantly reintegrate, although he's never been able to fix his condition.
However, he's recently learned of an artifact that can summon a Mythical Pokémon known as "The Original One" and grant access to the Hall of Origin. Perhaps this way, Red thinks, he can get his old life back.
It's time to return to Sinnoh.
I came up with this on the spot, lol 
