Optional addon to my sound replacer mod here: https://www.nexusmods.com/doorkickers2/mods/191

This removes the sound of the sniper rifle case dropping when the unseen sniper (First introduced in the mission: wood doors are easy in the FNGs welcome set of maps) fires a shot. 

Seeing as how they're hundreds if not 1000+ meters away from the assault team, it wouldn't make sense for them to hear it. 

Installation instructions. 

Go to your Tier One Equipment Folder

Ex: D:\Steam\steamapps\common\DoorKickers2\mods\TierOneOverhaul(v0.9.7)\equipment

Backup: firearms_rifles.xml

Extract this xml file into your tier one equipment folder.