Alright here's what I did wrong. What lionz said was the correct way of doing it. My problem came with the conditions I had to stop the sound.
My enemies had a death animation before the object was destroyed so I needed the sound to stop when they stopped moving. I had "is not moving" as the condition for the action "stop sound."
I made the condition "on destroyed" to test and it worked as intended whether I had 2 or 20 instances of the enemy. Even though the sound plays during their death animation, it properly ends the sounds as they should. So I made a separate event that had the original condition of not moving as well, and no matter how many instances I had, it stopped the sounds as it should, while also stopping it when the enemies had their death animation.
Hope someone can learn from my mistakes :)