As for this issue, it seems to be caused entirely outside the Aether. Minecraft is searching under our islands and finding huge dark spots, leading to it triggering the ambiance noise generator (cave noises). Our best bet is to just cancel the sound events all together in the Aether, but that isn't the cleanest solution since the Slider also uses the sounds.

The Slider would be in the dungeon dimension, not the Aether. Couldn't you cancel it just for the Aether, as you said?