Nice zombie shooter! Killed a good 63 zombies before I died.
I noticed a "crackling" noise from the rifle sounds if I held down left click. This is happening because you're cutting off the "shoot" noise from the first shot by playing the shoot noise from the second shot. The crackling is caused by huge jumps in audio waves. You can fix this by utilizing AudioSource.PlayClipAtPoint.
Nonetheless, awesome job!