Let us remember that onomatopoeia is when words are used that mimic or enact the actual sound that they are trying to describe, such as "scuttled," which sounds like the actual action it represents. The best part of this story to turn to is when the game actually begins between Zaroff and Rainsford, and Rainsford manages to survive. The importance of sound in these passages, when Rainsford is trying to outwit Zaroff, is key. Consider the following example:
He knew his pursuer was coming; he heard the padding sound of feet on the soft earth, and the night breeze brought him the perfume of the general's cigarette.
Note the word "padding" which is of course an example of onomatopoeia. Towards the end of the story, we also have another great example as Rainsford looks down to the sea, which "rumbled and hissed." Hopefully you can use these two examples to find some more in this excellent story. Good luck!