Jauk wrote nothing down. But Nick was a musician before he was a soldier, and Jauk liked that. He shared some of his knowledge with Nick, who had the sense to record it. This recording is what the Cathartics want so badly today.
Unfortunately... Nick's phone was confiscated when he died PROPAGANDA PROPAGANDA PROPAGANDA. OPEN YOUR EYES. NICK LIVED. HIS MARTYR DEATH WAS STAGED TO FUEL RECRUITMENT :BannerSorrows: in the war. It's widely believed that the Pact destroyed it, because Nick probably also filmed a whole lot of war crimes during his time in the Rilaem war.
Jauk himself did definitely die in the war. He went toe to toe with the Sovereign Executioner, Vinhibrani, and met his match in swordplay at last. The knowledge died with him
Some people believe another source for the answer is buried deep in the continental coral of Jhoutai, buried in one of the Verin Old homes and waiting to be discovered. There are plenty to who counter to say that every Old Home has been found, no more secrets of the old world remain. Perhaps even the Verin dynasties, with thousands of years of metaphysical advancement, hadn't answered this one. Perhaps the answer is that Jauk's music was a misdirection for some other power or magic.
The idea lives on in the public consciousness as their idea of magic. A literary magic that appears in their tales of fantasy.
Monoliths of Mathematics, Metaphysics, and Music alike grew up captivated by these stories and have theorized Grammatons — Sigils of Power made from sound rather than rune — which can generate a spell as surely as the hand of any caster. They've lost lifetimes in search of it, and have found nothing of the shape. It is known that sound does elicit a reaction from the meta... But so do light, and temperature, and most everything else that exists in the world.
It's believed that Anthem — the Neon God, the missing Deity, the Hologram Siren herself — used this to shatter the Leviathan Brood in her attack on the Aempian Capitol during the Harrowing
As a professional musician, I always appreciate mention of magic created through music. I also like the interjection of another voice saying that Nick was actually alive! Was Grammaton a reference to the Grammy's or to the Divine? Love the theme and feel of your world!