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

[AmemeAmeklin]

Actually, if you dive into the Source Lake, there are strange crystalline structures near the string of isles of Sullen.

Sorry to disappoint, but a mining leve from Crystarium tells us these are just giant extra pure salt crystals XD I got excited about those too, but then I got to hack at them later and was deeply disappointed.

One Ancient we talked to noticed (who had mistaken us for a child) noted that the Aether within us was WAY weaker than it should be (even for a child), and recommended we get a device to assist us in performing Creation magic.

As for this, the ancient specifically said our 'capacity for Creation' is nonexistent, so they sent us to beat up a cubus to harness its aetherial energy. Then they gave us an item that apparently was a concept matrix...and it was a lightning cluster. So a crafting catalyst was our reward. I assume this is partially a limitation to Emet-Selch's illusion (the merchant in Amaurot is also extremely limited, for some reason Emet-Selch saw fit to let them sell fishing bait?!) and partially a limitation to our own abilities thanks to sundering.

Concept Clerk: Pardon me for asking, but your creative potential is, um...relatively low, is it not? We are trained to assess these things, and I fear that you do not possess the ability to express this concept in physical form. It's almost as though you completely lack it, though that can't possibly be...

[...]

Here you are: a creation matrix that you may use to generate your own robes.

The concept is inscribed upon the crystal, which has been infused with the aether of the beast you brought me.

...What? Why are you looking at me as though you do not know how to channel the powers of creation? It is as natural as breathing─even a newborn babe has the instinct. Even if your potential is lacking, as one of our people, you must surely know what to do...

- Community Cohesion

I sort of figured based on this that crafting itself is a limited form of creation, since all recipes use elementally aspected crystals as catalysts to make something out of just a few items. The issue for the ancients would be that we sundered souls need materials, experience, and a pattern of sorts as well as the catalyst to actually make anything, and we certainly can't create fantastic beasts out of thin air. Now, with an actual proper concept matrix like what's seen in Anamnesis, it's shown that merely channeling aether into one will bring the concept to life. Bigger or more complex concepts require more aether, and therefore more than one Ondo has to channel into it to activate it. It doesn't necessarily kill the channeler, either. Comparing that process to Emet-Selch, who gave us an entire dungeon of monsters to battle with relatively little effort (I assume - we never actually saw him make it, so who knows how long it actually took him or how he did it), you can see why he would consider us weak and malformed.

Related to the topic, I surmised from the MSQ that there was more than one Sound.

Anxious Amaurotine: Have you not heard? Though yet confined to the lands across the sea, a terrible phenomenon afflicts our star. They are calling it the “Final Days.”

'Tis said it starts suddenly, a cacophonous keening from beneath the earth. The sound distorts all living things within earshot, and wrests from us control of our creation magicks.
Once that happens, all is lost. Fear, pain, despair...every dread impulse is siphoned from our minds and given substance: an eternal fall of fiery rain; an incessant spawning of nightmarish beasts...

None can point to the source of the phenomenon. 'Tis as if the star itself has fallen ill─as if a force inimical to life now festers and spreads.

'Tis only a matter of time until Amaurot, too, resounds to that discordant squall. You should stay with your loved ones, child... Stay with them...

- The End of a World

It seems from this that the sound would travel and cause havoc. Azem would've followed it, most likely, trying to put a stop to it...

This is mostly conjecture but I have had the impression from the Amaurot dungeon that the boss battles were originally fought by the convocation. I don't think it would be wrong to suggest, post 5.3, that Azem would have arrived in Amaurot (despite leaving their seat) and fought Therion with Emet-Selch + others.

The question remains as to how Lahabrea, Emet-Selch, and Elidibus escaped the Sundering when Hydaelyn was summoned, not to mention the exact timeline of summoning...I feel like Emet-Selch would've thrown himself into summoning Zodiark only after Azem ends up falling in battle, considering he chose to make an Azem fruit gummy in secret despite being tempered (I would assume these were made some time after the sundering, to raise the shards of other convocation members to their office), and he made seemingly a dozen contingency plans and even helped us out of the void despite being dead...but that's all baseless speculation for the moment.

Hmm, now I'm all fired up! Time to write some fanfic! XD