I was mentioning before (before the server lost a few messages a short time back) when playing your game that doors would not be opened by enemy characters. But I played it a bit longer and noticed that they do in fact do this, IF they are in a room where they have already seen you. That's not bad, but I can assure you that in real life you fire off a 9mm nevermind a big ass chain gun, you'll be heard through a steel door. lol

If I may recommend something; why not have a distance from which when you fire off your weapon, if there is an enemy within 'hearing range' they will come out of their default 'trance' and come to check it out? You could even have them pick a random spot from where the shot was 'heard' from to simulate the guy not knowing where the sound came from.

And of course you could then have different ranges for different weapons. ie. the big chain gun would be louder than a 9mm, rifle or a knife.

Just some gameplay thoughts.