The Sound is what bugs me the most honestly, I lean toward it being the fault of the ancients (that's the usual trope, they tread too close to the sun), not to mention we only hear of it from Emet-Selch's side who is notoriously pro-ancient. The fact that some ancients saw fit to split their existence into shards also makes me lean towards this route.
The possibility that a third existence outside of Zodiark and Hydaelyn also exists and is pretty exciting, but nothing of any sort has been mentioned like that so far, I think?
