The shooter uses a revolver with a suppressor. Revolvers can't be 'suppressed' due to gas escaping from the gap between the revolver's cylinder and the barrel.
Richard shoots Burton Reeves with a revolver that has a silencer attached to the barrel. Silencers do not work on revolvers, so the sound was unsurpressed.
A man shoots another with a suppressed revolver, but the sound is unsuppressed.
In the afterword, Alfred Hitchcock refers to Mrs. Kopeck (Lisa Golm) as "Mrs. Grant." ("Needless to say, Mrs. Grant was brought to trial...")