I've had a look through my sound files and the only zombie one I could find was located here:
C:\Program Files\Valve\Steam\SteamApps\youraccount\counter-strike source\cstrike\sound\ambient\zombiemod
It's called zombie_ambient.
Try downloading it here:
FPSBANANA > CSS - Counter-Strike: Source > Sounds > Player/Death > zombie_ambient
Then put it in the folder I mentioned above, you might have to create a few folders. See if that works.