Hi, this plugin was initially coded before subfolders were supported by rpg maker. It is a little complicated to change for this plugin compared to the menu theme plugin because this one allows for SE/ME/BGS/BGM. In the CGMZ_GameOver_playSound function, you could try replacing the first 3 lines with this:
const file = CGMZ.GameOver.Music[$gameVariables.value(CGMZ.GameOver.MusicVariable) - 1].split("/");
const type = file.shift();
const sound = {name: file.join('/'), pan: 0, pitch: 100, volume: 90};I did not test it for all environments but on initial tests it worked in my playtest