I do not consider that the Ancients' use of creation magicks was the cause of the Sound and the Terminus; Y'shtola notes that they had enough intrinsecal aether to do that without harming the Lifestream, and whenever they needed to do greater things, they resorted to their own life energy, like the researcher in Akadaemia Anyder who surrenders his very soul in summoning Quetzalcoatl, or the collective summoning of Zodiark. The sins of the Ancients, the way I see it, are more related to being responsible of the greatest catastrophe in mankind's history: the cycle of eternal war and mistrust. 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. Fear of the unknown (at this point it was unknown, because Venat and her followers knew not what Zodiark might do, and yet they were willing to, and they were well aware, doom their people) begets violence. And it has been mostly the cause of many, many wars; to put it in Emet-Selch's words: "A war without knowledge of the enemy is no war—it is mere bloodletting."

But I digress. What I consider interesting is something that is generally overlooked and it could be a clue about the nature of the Sound and the Terminus: the DNC quest, specifically the Kriegstanz and the Totentanz. The "dance of the damned" is negative feelings taking hold of people and controlling their bodies, and the Kriegstanz forces those energies out, and curiously, the forms they take are eerily similar to the Terminus monstrosities. Now, the narration of Amaurot mentions a passage of the Book of Revelations that goes more or less like: "But it was neither fang or claw, but our very sins, stacked to the heavens where they took root." The Ancients' society tried actively to get rid of any negative feeling, to foment peace and cooperation, but going by Azem's role, they were not strangers to issues and threats. Perhaps, subconsciously, they "stacked to the heavens" their fears until they burst and caused the Terminus and the Sound, which could be the collective suffering and fear of the Ancients throughout the ages that they cast aside. We would need further insight into the Ancient society, true, but to me that sounds (no pun intended) rather similar to the Totentanz phenomena.