As of right now the only things you will hear are any sounds from the kill animations (no vent noises, also the annoying Alien scream is gone too!), also you will hear sounds of the Alien when its killing NPC's. So basically you will only hear its sounds when its killing, kinda like the movie. For the best effect I would personally turn the music off and load a save where the alien is present for the mission or play a survivor mode map. Happy hunting.
This is a mod that im working on to silence the Alien's sounds and movements. Now eventually what I want to do is just silence the walking footsteps and possibly vent movement sounds as well, here in this video I have taken out all the sounds of the Alien to give you an idea of how this game would present itself if you couldnt rely on sound direction so much, this forces the player to have to rely on the Motion Tracker to get the best sense of direction of the creature. Why? Well in the original Alien movie you never could hear the Alien stomping around, it was always sneaking up on the crew one and a time with very little noise, making the crew have to rely on the motion tracker in a few scenes. Again I DO NOT want to completely remove the Alien sounds, I just want to bring a more active representation of how stoic the Alien was in the movie.