My understanding is that the Sound is coming back right now.
In 4.5, you had
Matoya's magics shorting out. A few people I've talked to have attributed that to the Light aether from the First, but light aspected aether shorting out magic is not an effect demonstrated in the story. What is? The Sound. at the end of 5.0 we learned that the Sound warped the magics the Ancients used.
Then, in 5.2,
Urianger heard a sound which ruined his water walking spell. In 5.3,
Alisae and
Y'shtola hear the Sound as well. These incidents are distinct from Thancred becoming tired due to his soul being divorced from his body.
In 5.3, the WoL picks up the soul stones of the Ascians, which records the moment of them summoning Zodiark. The dialogue of one of the crystals seems to suggest that one of convocation members apparently got screwed up during the summoning, probably by the Sound. Said convocation member is almost certainly Fandaniel, who seems to be very different from all of the others. Zenos wants to recreate the Final Days, and he's now working with Fandaniel. It seems pretty obvious that 6.0 is going to culminate in us confronting the Source of the Sound.
One of the reported trademark names for the 6.0 FFXIV expansion is "Forspoken", which means to curse or enchant or bewitch something or someone. That'd line up with 6.0 being about the Sound and its effect on the planet. It's even addressed as a sickness by Lahabrea in 2.0 (though obviously I'm skeptical of whether or not the Sound was conceived of at the time 2.0 was written, but Lahabrea's line will probably be retconned into that, just as how the Starshowers were neatly retconned in 5.2).
There is little reason to visit Sharlayan unless we need the best and brightest magicians and their aetheric knowledge to save the world from a metaphysical threat. That threat is the Sound. There are also parallels between the Sharlayans and the Ascians; they are both great magicians from a solitary city-state. The Amaurotines heard of what the Sound was doing to other nations, but refused to get involved until it was affecting them, and by then it was too late, and their solution was halfbaked. I'm guessing the Sharlayans will succeed where the Amaurotines failed by heeding the call of the WoL and the Eorzean Alliance and getting a headstart on working on the problem, and they will be able to use the resources of the Alliance to find and implement their fix.