Now I could have read this wrong, but I'd like to hear how it fits into this theory.
I wasn't really equating Ronka with the Ancients, merely stating that Ronka clearly understood the gravity of the information they had in their care. As we see through the quests in Rak-tika, the last emperor of Ronka went through great pains to establish the myriad wards, safeguards, and trials which were all designed to protect their great knowledge, which ultimately turns out to be the ancient murals depicting the Final Days. While the creation of the murals clearly predates the Ronkans themselves, the level of import that was placed on them can only be explained by the Ronka knowing what they represented, and wishing to pass that into the future.

As to how they knew, there could be any number of explanations for that. Emet or another Ascian, for one. Alternatively an Echo-user may have seen those events through interacting with the murals, or perhaps there was some sort of oral tradition dating back to their creation that explained the context. We don't know the specifics, but it's clear the Ronkans were aware somehow.

As Riastrad pointed out, the mural bears a very striking resemblance to Lavos. Which has me worried. I would be very disappointed if the Sound turned out to be yet another god monster final boss#4841. I'd prefer for it to be a metaphysical problem resolved through magical knowledge and pooling the resources from all of our friends, with Zenos simply just blocking the way to implementing the fix, because he knows that we would fight at our hardest then.
In the past I've suggested that the Sound may have been created through the collective unconsciousness of all mankind in the days of the Ancients, inadvertently amalgamating over time in the Lifestream through the use of creation magic until it gained awareness and attempted to end all life. And while I think this is at least as likely an explanation as the Lavos/Jenova-type theory, I do think Hydaelyn and Zodiark already sort of fit the niche narratively for being that sort of issue. But whether the Sound came from elsewhere or was a product of the Ancients themselves somehow, I think we can say outright that it's some sort of entity rather than just a metaphysical problem.