It's one of those suspended-disbelief accomodations that has to be made when playing a game... even in SOD or Breakdown 1, the number of zombies in the radius that could hear a typical rifle shot would be immense, to the level that it would make gun use nearly always fatal because an army of zombies would charge to the area. There is a complicated table of ammunition types and suppressor results at http://silencertalk.com/results.htm, and there are some interesting numbers there. The .22 rifle tested was 160.7 dB unsuppressed, with best commercial suppressor reducing that to 117.7 dB. That is the quietest result listed, from a variety of ammunition types. For comparison, standing next to someone using a chainsaw or jackhammer (1 meter or so) is rated between 100-110 dB. So, even suppressed weapons are very loud... the primary purpose is to push the levels down beyond the point where the risk of instantaneous hearing loss is common. Movie-level quiet weapons just simply don't exist, outside of custom-designed weapons using specialised subsonic ammunition- all of which results in vastly reduced range and impact performance. Also, for mechanical design reasons, revolvers and shotguns are typically not able to be effectively suppressed. Those few with some success have been custom-designed with noise reduction in mind, not something that can be added to any available weapon.