Just to clarify a few things 'fore I pass out:
The phantom Amaurot is taken from an exact point in time: according to the phantom Hythlodaeus (a.k.a. Emet-Selch), it's drawn from the eve of the Final Days reaching Amaurot.
Azem had already vacated his/her seat in the Convocation at that point, or otherwise wasn't present.
My general read is that Zodiark was simply too powerful for Hydaelyn to defeat in a contest of pure power (not surprising), so she had to sunder him to fulfill her directive; unfortunately being "the will of the star" this act sundered the physical planet and everything on it as an unintended side effect.
I'd agree the Sound was just a MacGuffin to precipitate the fall from paradise, but considering how detailed this game tends to be about the smallest things, and in the Announcement Showcase one of the questions posed was specifically "What [caused] the Final Days?", I don't think that's the case. (And, being inquisitive by nature, I'd be super disappointed if it were.)