The supersonic crack of bullets overhead is loud, as loud as a child's cap pistol.
Also, you hear it before you hear the sound of the muzzle blast.
Good training to be on the receiving end of weapons fire.
Suppressors and sonic booms are an interesting subject. You hear a sonic boom relative to what it passes, not from where the bullet came from. Basically you'll hear it crack when it passes you, and you'll hear the echo of it's boom as it passes other objects. You still hear a gunshot, but it doesn't sound like a gunshot and you can't pinpoint it's origin. Silencers are also much more effective outdoors than they are inside. I'm not exactly sure why that is, but it's true.
Also, you hear it before you hear the sound of the muzzle blast.
Can you still tell the direction it came from, or is that much more difficult when the weapon is suppressed?