The game still seems to have "wake up zones"
If you can somehow draw a bead on a zombie from like 3 rooms over, you can pretty much fire a rocket launcher and they won't wake up (except for waking from the damage). other times, you can be 10 meters away and reload a crossbow and its enough to trigger them.
From what I can tell, there seem to be hard and soft borders to sound/waking when clearing a POI. Sometimes you're JUST outside of wake up zone, and you can take zombies out from fairly close without sound seeming to matter much at all....as long as you're sneaking at ALL, it seems to say "good enough!" Thats the soft border. You can often kill enemies this way so long as you aren't in the "area that counts" - case in point? take Factory_03, the new pharmaceuticals factory....if you walk in the front door to trigger the zombies spawning, then walk back out, but can draw a bead on some of the zombies inside from several meters outside the building....no wakeup (presumably this is to keep you from aggroing zombies in a totally unconnected area that you just happened to get close enough to spawn - as they'd then start roaming around breaking doorways and ruining the path through the POI). If you move inside the building and shoot, or sometimes if you're RIGHT up against the wall and hit the wall (hence the old tactic of running around the outside of a POI hitting the walls to wake everything up) they'll sometimes wake.
Other times, there seems to be a wall between you and the zombies where they flat out won't wake up until you make noise. Take that same Factory_03 POI....as you go through the POI, there are several vertical lifting garage-type doors with science zombies RIGHT on the other side. Until you open that door, there's almost nothing that will wake them up. Once you open the door, you can actually be standing out of sneak mode, right up against the door and open it....you'll be standing unsneaked about 2m away from a sleeper and they won't react (until you move unstealthed or shoot or something). Thats one of those hard borders. It doesn't seem to matter what you do until that door is opened....you can even kill the trap zombie that drops from above outside one of those garage door areas with an M60 and everything on the OTHER side of the door is still dormant.
This is the reason that people say "why bother improving stealth when I can just bash down a wall and take the zombies out from behind/sides without them ever waking up?" - because those wake up zones seem to be coded to roughly the room shape, so bashing the wall down lets you attack them from an area they were never meant to react to sound from.
In other words, call it a quirk of the programming. A consequence of the decision to compartmentalize the areas in POIs and not have EVERYTHING in a giant POI wake up and start roaming around the instant a gun is fired anywhere.
