Game version: 0.1.11
When monsters gain the exact amount of XP needed to level up, it plays the level up sound repeatedly, and gets pretty loud when multiple monsters level up at once.
(Apologies for the very short video, for some reason recording wasn’t working correctly and started late)
Edit: Chanced upon the bug again and got a second longer recording. This new recording also shows how much louder it is compared to the rest of the game volume.
Edit 2: I was probably wrong about what caused it, since I got the same circumstances again when the four monsters got the exact amount of points needed to reach level 4 and the bug didn’t happen. No level up sound spam, and no loud noises. The version is still the same as the original bug encounter, and the second encounter, version 0.1.11.