Oh, why is that? Our band of heroes is small and routinely saves the world from world ending threats. Why does the size of the group propagating the disaster matter? For that matter, the Convocation of 14 is a small group, and they over millenia have destroyed entire worlds in the name of Rejoinings.
Also, I think you misunderstand the idea behind the Sound. The Sound itself only had the power to drive the Ancients mad. Once the Ancient's own creation magicks were running amok causing their worst ideas, nightmares, and dark fantasies to come to life it had but to spread. Think of it like a germ. It infects one individual. That individual goes on to infect more individuals. In time, a pandemic ensues, and then what?
By the time Zodiark was conceived, they weren't just stopping the Sound. They were stopping the entirety of all lifeforms with creation magicks worldwide. Also, it's actually still up for debate as to whether or not Zodiark stopped the cause of the Sound, as his main focus was to stop the apocalypse it had wrought. It actually wasn't explicitly stated that he stopped the source.
We can conjecture, too, that Zodiark did this without killing a single Ancient that wasn't part of the sacrifices required to first create him and then to undo the apocalypse and its damage. The reason he was not a long term solution is because every world bending action he took required sacrifice, but also because the Ancients never fully learnt the cause of their apocalypse. This meant more apocalypses while the body of leadership had become zealots for their man-made god.
Hydaelyn was made specifically to combat Zodiark, and until they tell us otherwise, I'm still of the belief that her enervating power was taken too far, and that the Sundering was incidental. That it stopped the Sound only incidentally, and it was actually a horrible mistake.
As for Fandaniel needing a ton of power to bring the Sound back or what have you... well that's going to come from the Ancient's own gods once they're dug up. We're going to get a gravy train of all the missing puzzle pieces here at the end of November, but I firmly believe that Fandaniel's twisted mind comes from being related to what the source of the Sound is actually. He doesn't really need the same exact proportions as the original Final Days to pull off another Armageddon. We are still sundered beings after all.
Consider this: If the Ascians had wanted to end the world, then they could have. They could have at any point in their history. One now does, and he has all of the old knowledge to do so. Knowledge is power, or so the saying goes, and knowledge is going to be a theme in Endwalkers, I think.