I think the lack of speech for Corvo probably has something to do with freedom of action you are given for completing the game. It would be exceedingly difficult to write lines for the guy that would still make sense regardless of whether he'd been sneaky pacifist who showed mercy on all his targets or a raging berserker who kicked in the front door and murdered everyone who moved, including helpless civilians.
You'd have to record like 3 or 4 versions of each line of his dialogue to cover each of the different emergent personalities he could have based on your actions. That's a lot of work and money to do, it's a lot easier to just make him silent and let the player imagine an appropriate response that fits their play style.
You'd have to record like 3 or 4 versions of each line of his dialogue to cover each of the different emergent personalities he could have based on your actions. That's a lot of work and money to do, it's a lot easier to just make him silent and let the player imagine an appropriate response that fits their play style.