That's correct. It's because of Cosmo Canyon. You can hear the planet there. And Bugenhagen and AVALANCE gained the ability to hear what the planet says. I've just mentioned the thing with the forgotten capital tto make it easier because you learn some abilities from the cetra there as well - well, Aerith and Bugenhagen. Aerith and Bugenhagen learn to understand how the White Materia works and Bugenhagen learns how to use this music box think to activate the waterfall.
You can also hear the planet way up on the Highwind.
It's hard to say if they gained the ability to hear the Planet-
1. Who's to say it was or wasn't 'crying' before FF7
2. It's unknown if other civilians (non-player characters) can hear the Planet.
But Bugenhagen certainly taught them what that noise is. Bugenhagen learned over time, from studying various things. I don't think it's ever outright stated how he learned what the Planet's cry sounded like, so yes, it's plausible that he researched Cetra sources.
I have to re-read about the White Materia, because I do not recall that info off-hand. However, what I do know is, the only reason Bugenhagen knows anything about the "music box"/waterfall is because a scientist wrote a note in chalk by the Cetra writing. I believe he even says something to the effect of, "Oh I can't read the writing of the Cetra!" (I'm perfectly willing to go quote it, if you'd like.)
I am sorry, but that's not just speculation. I can read the Ultimania Omega as well - it's just that most of the people don't accept what you write when you say you were the one who has translated it because then you get the answer "Yeah, of course...you can tell us everything as long as we can't read it.". So that's why I can assure you that most of his translation and résumé is very good. I've already said, a few mistakes are in it (of course - japanese is hard to understand), but the rest is not bad.
But it IS speculation. The quote you took was clearly from the author's "interpretation" section. I re-read the section many, many times to make sure I didn't miss anything. Nothing in the original, Ultimania translation says anything about a normal human of non-pure Cetra ancestry being able to become Cetra by doing anything.
I think the idea that all people on Gaia have Cetra blood in them (due to the fact that the surviving Cetra lived among humans) is a really interesting theory. I think it's plausible. I don't think it's plausible enough to say that therefore AVALANCHE can hear the cry of the Planet due to that. And, again, there is nothing from the actual Ultimania text that backs up this theory.
I also think it needs to be stated, there is a difference between AVALANCHE hearing the cry of the Planet, and Aeris listening/talking with the Planet.
AVALANCHE hears the cry, but they really cannot say what exactly it is the Planet is trying to say, except that it's being hurt. Nor can any of them converse back to the Planet. (I kind of think of it like when a baby cries, you hear it crying... but you don't necessarily know why it's crying, just that the baby is in distress.)
Aeris, in a limited fashion, can sometimes hear and interpret what the Planet is saying. She can also, at times, talk back to the Planet -- most famously when she prays for Holy.