Only a certain number of sounds can be heard at the same time. As of writing this article, the limit is 64. The engine prioritizes sounds by their mixed volume, so for example a zombie growling next to you is higher priority than a quiet footstep 20 meters away. Inaudible far away sounds aren't even considered. Your own gun manually gets higher priority than everyone else's so that rapid full-auto fire doesn't cut out.
This can become problematic if a mod has very long gunshot sounds. Some gun mods have 10+ second long gunshot clips in order to realistically capture the tail, but which quickly fills up the limit. Most commonly this problem is reported with modded guns, in which case the mod creator should reduce the gunshot length.