He means- the bullet (travelling faster than the speed of sound) will potentially kill the person if, say, a headshot that were instant lights-out; before the sound wave (travelling *at* the speed of sound) would arrive at your ear. Thus, you may not even hear the horn(as the victim), even though it did make a sound. Most ammo is supersonic(faster than speed of sound); subsonic ammo, which is common in most suppressed applications due to no "crack" of breaking the sound barrier, would allow you to very briefly hear the horn, since the bullet is travelling slower than the sound wave.