https://github.com/BananaSquares/Forge-Mod/blob/master/src/main/java/mod/trianglesinpoo/mcores/ModSounds.java#L12
new SoundEvent(new ResourceLocation(Main.MODID, "music_disk/equilibrium.ogg")
Just change it to something like:
new SoundEvent(new ResourceLocation(Main.MODID, "equilibrium_disk")
or just simply "equilibrium" as long as it is same as https://github.com/BananaSquares/Forge-Mod/blob/master/src/main/resources/assets/mcore/sounds.json#L2
You should really make the names more consistent.