Zombies both scream with real audio and with hidden attraction sound that can be "heard" by other zombies.
Details:
Use NORMAL Zombie hearing settings.
With every zombie you kill there is a 1:100 chance they would scream and attract zombies from 200-300 tiles away. There is also a 'Rare' version added that has the chance ratio 1:300.
It is a single sound event so it's "severity" depends on the zombie memory settings as well - Its the one in vanilla sandbox settings that determines zombie memory.
If you are in the woods or inside the house then there is a good chance the scream won't be heard far away.
It only affects these zombies who are already spawned in. The ones who are in "cloud" so to say(outside the white box in preview video) will not be affected and will just migrate on the edge of your white box, spawn in and start their default movement patterns.
It will not create any extra zombies into the world but rather migrates them closer to you(screamer location).
Works in both SP and MP and can be added mid game.