and the fact that you may not be in danger if you hear a machine gun shooting, but will be in danger if you hear the bullets thudding into the ground.
Ummm. The M60 machine gun for example has a muzzle velocity of 953m/s and an effective range of ~1.1km. The speed of sound is 343m/s. If the machine gunner does his job right then you won't hear anything at all. You'll just be dead. Long before you would be able to even hear any kind of firing. That dev's got the wrong order. What you would notice first if the gunner misses is bullets thudding into the ground and only after that would you hear the gun shooting.
P.S. for the smart cookies: Yeah, I know that the bullet will slow down as it travels but it will be a while before it drops below 343m/s so unless you're being shot at from a far distance there'll be bullett impacts first and sounds second.