I've never heard the zip of a bullet going by
You don't have to go into combat to hear the zip. Just go to a high power or F-Class match and pull targets.
You get the zip and after a bit you'll learn what "zip" is going through your target and which one is on the target next to you.
Not the same as seeing little green tracers flying over your head but you'll know what a super sonic bullet sounds like.
It just means the bullet is going faster then the speed of sound. Meaning it will get you before you can hear it.