The biggest problem was to find information about all the animals in the zone. Wolves, birds, many birds!!!, frogs, dogs and many more... don't forget some spooky sounds for the night time!
Overall you will hear much more birds, like robin, sparrow, cuckoo, swallow, tit, blackbird and what ever I will find and which is present in the depth of the zone.
Pripyat will sound more like a ghost city. You will hear cars far away, passing by, voices from conversations, children play and laugh and other noises which will echo in the streets of this city.
In the dead land, known as garbage, you won't hear many birds. A crow here and there, but no other birds. But there will be some metal noises from the garbage lying around like it was in the old builds of the garbage.
The marsh won't have any frogs, which will constantly spam noises. There will be frogs, but there are variants of them. Several water or swamp birds, birds taking off, or landing, sounds from geese far away, mosquito's, swan, movement in the bushes...
For laboratories there will be many sounds from labs, like machines, bubbles, humming electric devices, weird voices, laughter and some more nice noises, which maybe help to wet your pants. :D
The soundtrack from Clear Sky will be expanded. There are more music tracks for the new levels and also for some old ones.
Be prepared to listen of a quite more alive zone then before. :)