Silencers on military firearms are really intended to modify the sound of firing, rather than mask it entirely -thus making it less obviously a sign of an attack in progress.
Today’s invention is a digital recorder and loudspeaker which is coordinated with the pulling of a trigger.
When a shot is fired from a ‘silenced’ weapon, a very loud masking noise is played. This might be the slamming of a door or the barking of a dog, depending on the current context. For automatic firing, a naturally repetitive sound would be selected.
Recordings might even be made of ambient sounds in the current locale, in order to avoid drawing attention to the attack.