When you are inside the bunker you get some sort of inside effect, like when inside a house for example (weather is not as loud, animal sounds are more quiet and the most noticeable: inside gun sounds). As far as I know this doesn't only impact how you hear gun sounds but also how others hear it, and how far they can hear it from.

The problem is that as soon as you get on an 'outside object' the game thinks you are outside. One of these objects is like the wooden box not far from the entrance in the middle of the corridor. If you walk on top of it, and shoot a gun for example, you can hear that it makes a different sound than when you fire from 1 meter away but standing on the ground.