Screw a silencer onto a pistol and it goes 'pfft', a discreet little cough you could fire in a quiet room and nobody would hear. Spies and assassins do it in every film. Silencer on, gun silent. That is the whole point of the gadget, surely.
A silencer, properly called a suppressor, does not make a gun anything like silent. A typical gunshot is around 160 decibels, loud enough to permanently damage hearing instantly. A good suppressor knocks off roughly 20 to 35 decibels, which still leaves the shot at around 120 to 140 decibels, comparable to a jackhammer, a loud rock concert, or an ambulance siren up close. That reduction is genuinely worth it for protecting shooters' hearing, which is mostly what suppressors are actually for, but it is nowhere near the movie 'pfft'. The cinematic version is pure Hollywood foley work; real suppressed gunfire is unmistakably the sound of a firing gun, just somewhat less ear-splitting. So an assassin quietly picking people off in a crowded building without anyone noticing is fantasy. Everyone in the building would hear a very loud bang, just one that did not also deafen the shooter.