That's a hollywood myth. Some sound guy's probably come up with that sound late at night in a soundlab somewhere, it's featured in a few fancy movies and now everyone thinks that the noise bullets make. I think subsonic rounds make a sound similar to that, but supersonic rounds don't.
Here's a useful quote written by someone who's better at explaining this than me.
QUOTE(http://guns.connect.fi/gow/highpow.html)
The Supersonic Crack
Any projectile moving through the air at a velocity greater than the speed of sound (332 to 340 m/s or 1,089 to 1,114 fps in dry, 18 C or 65 degree F air, depending on who one listens to) will create a supersonic crack. Temperature, humidity and atmospheric pressure variations play a role in raising or lowering the speed of sound by a small percentage. In a firearm which lacks a substantial muzzle report (being fired over an open field) the sound resembles the loud tearing of a bed sheet.
Two sounds are actually created, one from the front of the bullet, and one from the rear. Near trees and buildings the sound waves come back as a distinct crack or pop each time the speeding bullet passes some object with a vertical, reflective surface. Once the muzzle report has been diminished the supersonic boom becomes dominant. Curiously, the sounds will now appear to come from the target area, rather than the rifleman's position. Sound moves through our atmosphere at a relatively fixed rate. A sound wave will typically strike one ear a bit before the other.
The human brain is capable of detecting the difference in time between sound impacting one ear and then the other in an increment of as little as one/six-millionth of a second. With time and practice we soon learn to use this ability to pinpoint the source of a sound very accurately. Because a suppressed muzzle report is relatively quiet, the uninitiated will automatically home in on the loudest sound, which in this case is a sonic boom reflecting from the target area. The intense, sharp sound of the bullet's passage will seem much louder than the muzzle report to someone close to the flight path. Indeed, a rapidly moving .308 bullet will sound louder than a .22 LR pistol, to someone who is positioned a few feet from its flight path.
Smaller diameter bullets make less noise than larger diameter bullets. Supersonic is supersonic. A bullet traveling 366 m/s or 1,200 fps will make about the same noise as one traveling 1220 m/s or 4,000 fps. Projectiles that are .308 inch in diameter will be somewhat louder than .223 bullets. There is no technology which can remove the sound of a supersonic projectile, no matter what claims are made to the contrary.