Darth Smeg said:
Outdoors you can hear a gunshot for miles, but in the underhive? That place is a maze of crumbled ruins and working machinery. I imagine the "walls" are groaning with sounds of pipes adjusting to pressure changes, gears moving, the rythmic thuds of distant factories and generators sending vibrations through the metallic support-structure of the hive, etc etc.
Basically, it will be a cacophony that will drown anything further away than, I dunno… just as far as you'd like
Add to that the fact that by the time you heard a gunshot, it has either gone past you or hit you. Unless the shot comes from very far away, in the order or 1000+ meters, you won't ever have time to reach to the noise, much less move something as large as a sword in front of something as small as a bullet. Essentially, it's impossible.
Edit: Ah, I've made a mistake. Any gun capable of hitting you from more than a kilometer away will kill you before you can hear it. Considering the fact that many of those weapons fire a bullet that is literally faster than the speed of sound.