Start OpenIV 
2. Enable Edit Mode. 
3. Go to x64/audio/sfx/WEAPONS_PLAYER 
4. Extract the "Better Handguns Sounds Effect" file to your desktop. Open it 
5. Open "WEAPONS_PLAYER" 
6. Drag the three .oac files in it to your WEAPONS_PLAYER directory 
7. Click "Edit Mode" again then exit OpenIV 