That's what I have done, I extracted all the sound effects, all 700+ of them, and listened to them 1 by 1, twice over now. I can't find it.
I can only imagine that the sound might be generated on the fly by the game, or maybe it's made by combining multiple effects or something..
It might be easier to disable music and run the game, finish a battle, and record the sound with Audacity's "What I hear" option, but I can't get the game to run on my laptop which is all I have access to for a while.