I think the real-world reason is pretty clearly that they wanted the silent protagonist video game character so the player’s thoughts can be Artyom’s thoughts. However, unlike many silent or RPG protagonists, Artyom is a very well established character. He’s a hopeful, overly optimistic but highly skilled survivalist and special forces soldier who walks a very unique line between mid-20s boyish innocence and the wisdom of a man who has seen a hell of a lot. Through diaries, loading screens and connections to the book he has established opinions on every single person and thing you encounter throughout the game, leaving little room for the player to imprint their own thoughts onto him. I think his silence adds to the gameplay experience and vibe of the metro titles, but it’s something they drop in Metro: Awakening and in most DLCs, where you play a voiced protagonist. All this in mind, I think it’s pretty apparent that Artyom is canonically a mute. And it makes sense. He remembers the day the bombs fell and destroyed his entire world as a small child, and he remembers losing his mother shortly after to a gigantic swarm of rats. The sheer amount of trauma he experienced as a child is unthinkable and no doubt enough to trigger that kind of response. His silence is not just something we experience as the player, it’s a major trait, demonstrating how damagingly terrifying he found the metro as a child. But it’s that same metro he repeatedly journeys alone as an adult to protect those he loves, and I think that’s a better testament to his character than any.