It's really just built-in since that last update the other day or so ago. Zombies have idling noises, instead of just roaming around silently until they snarl when they see you. It's moans and groans and stuff like that, the jumper has a insectoid sound, but you can hear it quite loudly from a fair distance away. When I was doing my mission in the wasteland I was constantly aware of zombies that sounded like they were in the building with me, when they were actually outside and across the street. Should be automatic for you if your game is updated.
Yeah... the new sounds from specific zombies might seem to be ambient sounds. The zombie in the biohazard suit has a moaning sound that was just recently added. Maybe that or the mutated zombie sounds (not sure if I've heard that one yet) are what are being heard. I haven't noticed any new ambient sounds, but I generally tune those out and rarely notice them, so that could be why.
