I've played all the other games and the repetitive sound problem was always here, you can even find trace of discussion on the net that are years old.
Fact is, this game got music file in a folder, and you can "edit" them, but the FX sound are stored inside some other file i think.
Maybe the Dev. know a way?
-----
EDIT:
Seem i found the problem.. in the folder C:\Game\King's Bounty - Warriors of the North\data\ there is a file called sounds.kfs
This file contain all the FX reproduced in battle. The much "noisy" FX are labeled [name of the creature]_rare.wav and i think the Dev. mean to reproduce that sound only in rare occasion.
The game play the sound every 2.10 sec. instead, so is really annoyng
You can use WinRar to extract the file content to a new folder called sound, take note of what unit make the most annoyng FX and replace the sound with another, or ever edit and reduce the volume of the FX.
I still don't know how to "repack" the file so the game will work after the edit.
-------------
EDIT 2:
Ok, you just need to edit the "problematic" file, adding a .BAK extension to it, then repack the sound file using WinRar at fastest compression.
For what i've seen, the problem is with "_rare_" sound, so search them all and rename using the .BAK extension. Or you can simply note the name of the unit you want to edit.
Also, the game will give you an error message when is started, saying that sound file are missing: after about 1 hour of play, my game as crashed during battle, but none of the troop FX i've edited where present at the battle itself. It may be a random crash then.