I've noticed that when I'm being shot at the super sonic crack that the rounds does, is fairly low in volume. Compared to Arma 2 sound

I know that this crack is quite loud,and when under fire it's what causes the most fright in soldiers.

Maybe you could increase the decibels (volume) of rounds going past you, so that you are actually spooked by them, and through experience you can overcome that initial fright.