Been thinking about this for a while now while brainstorming a writing idea, and figured I'd look back on previous Final Fantasy and FF-adjacent games for possible influences for what the Sound may be.
Final Fantasy 4 and Final Fantasy 4: The After Years
FF4:TAY brings us the Maenads, half-Lunarian clones of Rydia sent from the True Moon to terrorize the kingdoms of the Blue Planet, shackle the Eidolons, and retrieve the planet's crystals for study. Their method of travel between the True Moon and the Blue Planet and Red Moon were meteors that inflicted toxifying wounds upon impact. Near the end of the game, the True Moon begins raining down hundreds of these meteors onto the Blue Planet before steadily approaching for impact itself.
Final Fantasy 4 also has Zemus, who was able to invade the mind of a young Theodore Harvey—better known as Golbez—and brainwash/control him. This despite the fact that Zemus was sealed within the Lunar Core. There's also the Lunar Eidolons, most notably Lunar Bahamut.
Final Fantasy 7
FF7 brings us Jenova, who came to Gaia upon a meteor that inflicted a gaping wound to the planet in the form of the Northern Crater. She then infiltrated Centra society, posing as the friends and loved ones of her victims to get close to them before transforming them into monsters. And while she was eventually defeated, her presence led to the near extinction of the Cetra and the downfall of their society.
Chrono Trigger
Lavos who, much like Jenova, arrived in the form of a meteor. Like Jenova, Lavos's impact on the world was felt immediately, as the world fell into an ice age that seemingly wiped out most surface life following his arrival. Unlike Lavos, he largely went under the radar after that, only surfacing as 65,000,000 years because of the kingdom of Zeal's meddling. Of note is Queen Zeal, whose drastic personality shift from a kind and caring mother to a zealous fanatic willing to sacrifice the entire world for Lavos is very reminiscent of the tempering process.
The final battle also has certain characters speculate that Lavos's life cycle isn't just the result of an out of control apex predator, but a calculated move from an intelligent life form to harvest a planet's lifetime worth of genetic material. There was also a sequel, but getting into that would require another post entirely due to how damn weird it is.
Final Fantasy 8
The Lunatic Pandora is a massive, towering structure housing and harnessing the power of the Lunar Crystal. The Lunar Crystal will, under the right conditions, cause monsters living on the moon to gather up into a giant ball and plummet to the surface of the world. This meteor (I feel like I'm seeing a pattern here) inflicts widespread devastation on a global scale while also wiping out all life near the impact crater, natch, and is also the entire reason monsters exist on the planet's surface. This phenomenon is known as the Lunar Cry.
Moon's haunted, basically.
Final Fantasy Tactics
It doesn't really belong here on its own, but I'm bringing it up due to the way FF14 adapted its final boss: Ultima, the High Seraph/the Angel of Blood. Specifically, it's an alien life form that arrived on the world in an unknown time period. All that is known about it is that the ruin and devastation it wrought was so severe that those who survived its wrath began to fear and worship it, resulting in it gaining a will of its own through becoming a primal. As the history of the world prior to the Sundering was lost to time, however, it's unlikely that Ultima in particular was the source of the Sound unless the knowledge of its existence was passed down by the Ascians themselves to her would-be summoner, Saint Ajora.
Also, it'd be kind of underwhelming for the source of the Sound to be something we've already defeated.
I think the most likely candidates for a "this is a reference to a previous Final Fantasy" would be FF7's Jenova or FF8's Lunatic Pandora, depending on whether or not Fandaniel's plan has any relation to it. Perhaps it's even both. The whole "the Sound weaponizes the Ancients' magic against them" bit is comparable to Jenova's infiltration/transformation strategy for wiping out the Cetra. And the towers and Lunar Bahamut may be the result of the writers doubling up on references in the form of moon shenanigans. Lavos seems unlikely due to his modus operandi. The Zemus analogue here is Zodiark, which would make him being the source impossible without some form of time travel (please no). The Maenads and the Creator are likewise unlikely candidates for the same reason as Lavos.
So... I'm thinking Jenova, or a Jenova-like entity, with Fancy Dan's plan bearing shades of the Lunatic Pandora and the Lunar Cry.