I kinda wondered this myself, because I was the guy who feature-requested such a sound effect be added, but the circumstances seemed to have changed.
I suggested that a sigh or a groan of some sort should follow Beethro slaying 15 monsters with no peaceful interupitons.
I don't know the exact numbers, but I think now its like the sound is triggered after x number of no-kill-moves that occurs after a chain of n consecutive kills.
I think the number of peaceful moves is counted so that the sound doesn't trigger twice during a chain of say, 30 monsters, or if there happens to be an itty-bitty gap in a chain of monsters. So if there's a gap less than x, between kills, the number of kills continues counting.
That sounds confusing!
Or perhaps the sound plays if, after the chain, there are no monsters a certain distance from Beethro.