Unlike a bullet...sound does not travel in a straight line. He is also been firing in areas with traffic all around and people UP to the point of realizing a shooting has taken place...have been listening to radios, road noise and other things...living in a major city I can tell you that being "enclosed" in a car, traveling you will hear LESS of a noise...as for "by standers" the sound of a gun being fired up to 1000 ft away is NOT like a fire being fired while standing next to the shooter...also we have NOT heard of people reporting the sound of gun fire...
URBAN noise is the sniper's friend. I wouldn't be surprised to hear he has made a "silencer" easily made from a gallon size plastic soda bottle for example...IF he has access to a metal shop..he may have made one...