"had the murderer had "suppressors," it would have taken longer for the police, and the people to know what's going on"

I doubt that.

At ~400 yards, people downrange are probably going to hear the "crack" (sonic boom of a projectile moving at 2-3 times the speed of sound) at least as loud as the muzzle blast of an unsuppressed rifle.

Around my house (within 1/4 mile), people shoot all the time. I often hear the "boom" of a large-caliber handgun. I almost never hear the "boom" of a rifle - I hear the "crack" of the bullet moving.