I have a hard time accepting the interpretation of the sound as seen in this thread: A literal, external threat that lead to the destruction of the Ancients. I don't feel there is much, if anything, in the game to strongly suggest that to be the case and I find it very hard to reconcile such a development with the general themes and revelations seen in Shadowbringers.
I am much more inclined to interpret it as powers growing out of control and believe the text in the game fully supports this. What we know definitively about the Ancients is that they had essentially divine creation magicks at their disposal and an advanced, orderly society that enjoyed debate and philosophy. We are also told that there was a "sound" that rolled or moved across the lands until it finally reached Amaurot.
The term sound I believe to be chosen deliberately because it represents an immaterial threat. There was no being that compelled the Ancients to go out of control, it was an invisible force that can only be theorised to be an entity at all. This theorising would be exactly the cause of the Fall and a very poetic way to end this type of civilisation.
To put it in a simple timeline of events:
- the Ancients lived for a long time, creating many things with their magicks
- as time progressed, these creations gradually grew more complex, as did their society
- eventually the process of creation went from a sort of natural course of being to something they deliberately studied, analysed and regulated
- these studies slowly fostered a subtle but widespread fear
- eventually a single mishap (the original "sound", if you will) could cause a chain reaction and echo across the entire world
In a society that is founded on the power of producing things out of thin air, philosophy in its broadest sense is a direct existential threat. As they studied the theory of creation they would inevitably confront concepts such as "the most horrific thing imaginable" and "unimaginable suffering". At the same time, dealing with this power in an organised fashion would produce meta concerns. What if someone else creates an eldritch abomination? What if that initial incident then spurs someone else's imagination? What if thinking that such a chain reaction could happen in fact is another start of a different chain reaction? Having any of these concerns at all is an immediate risk factor.
All of these theories could create an atmosphere of terror that eventually leads to complete loss of control over the creation magicks, doubly so once the first widespread incidents begin to happen. We know that Amaurot was experiencing despair before any harm befell it.
There was no entity that influenced the Ancients, nor did the planet itself rebel against them. It was simply a natural conclusion of their way of life combined with their inherent powers. That is precisely why their solution was to create Zodiark: To control them and prevent them from bringing about pandemonium.