Zayford would be the one to ask about this, really, but I'll try to help. Before, I don't think there was any limit on the number of bullets that could be whizzing at any time. That required huge numbers of nearestObject calls, which can cause lots of lag.
Supposedly, no more than 37.5% of bullets will ever make the whizby sound as they pass the player now. That is a direct quote from the script. Basically, there is just a random chance that each bullet will make the sound. This cuts the number of nearestobject calls down, reducing lag.
What I would suggest is that you set the ECP_max_whiz_sounds variable to something really high, like 50. I'm not really sure if that will increase the chance of hearing the sounds, but give it a shot. Otherwise, ask Zayford about it.