Uhh ok it's been awhile since I played Drakengard but I remember that no he was not mute not at the start of the game. His voice was the price of forming a pact with the dragon you can hear him talking to her he says what say you a pact or death. So no he's not mute his voice was the price for forming a pact with her its a random thing for every one in that game who makes a pact.