What exactly do we know as of 5.3 about the sound that was heard before Amaurot fell?

[DPZ2]

Zodiark's summoning upset the balance, aye, but I would argue that was the only solution they had for what it was coming, and it was Hydaelyn's summoning what caused the first war in history, because fear had taken root in the Ancients.

All we know of the Ancients is seen through the lens of the last final survivors of that civilization. To say 'the first war in history' has to be taken as the hyperbole it is -- we know nothing about the history of the Ancients.

There may well have been tens of thousands of years of utter chaos, with war after war after war before the Ancients became what they were in Emet-Selch eyes. The highly structured civilization we see in Amaurot is the culmination of what went before. It did not drop into existence fully formed.

It is entirely possible that the 'Sound-heard-round-the-world' was akin to the Warring Triad in the Allagan Empire. Something to be contained. Something forgotten as the world moved on after the Allagan's civilization was no more.