Research for the Zombie audio

http://www.youtube.com/watch?v=_npS_-NfxWk

As you can hear in this video of all the different  left 4 dead zombies sounds there are many different effects used to create the many different types of zombie sounds that are used throughout the game.

http://www.youtube.com/watch?v=90LgJXVnpNQ

Here are sounds taken from call of duty black ops zombies, as you can hear by listening too the sounds in this video is the same with left 4 dead the sound clips are heavily edited to make them sound not of this world.