>All this talk about NPCs interactions not being realistic.
There's always a possibility of a reveal, that Freeman simply doesn't talk out loud anymore. He's so used to stalking corridors of Black Mesa, strung up on adrenaline and paranoia, he subconsciously avoids making any unnecessary sounds. Not to mention he only had two proper sleeps in over a week of non-stop active combat, and one was a concussion. Hell, he's met Nihilanth and G-man, both of which seem to be capable of psychic/telepathic communication. Who knows what effects does that have a on a human mind. Could even be a deliberate action from G-man.
So while he used to speak before, now he simply thinks aggressively, unable to realize no sound is actually escaping his lips. Maybe he's even permanently mute now from all the trauma. Or maybe when he realizes this, and tries to speak, he can only make extremely stuttering noises. PTSD is helluva drug, and constant supply of splattering alien juices doesn't help.