Can guards hear it when NPCs fire their weapons at the player? The constant stream of guards arriving at the scene after an exchange with an NPC suggests that the Noise Suppression mod is made useless due to NPC fire.
They're aware of all sorts of sounds.
So make sure the NPCs dont get to make any.
And if a NPC fires back at you, you did something wrong to begin with
Also, Audio Mask and Noise Suppressor are best on high dps weapons.
The usual choice is Audio Maks on Executioner and Noise Suppressor on Tormentor.