How about the Kiai:
"Ya" "Toh" Unn (non-verbalized)" and "Ehii"
They contain A, O, U, E, and I and O-sensei used them martially, not just religiously. They too were passed down in Doka (to all who care) and Kuden (to some at least).
Just as A and O were specific referents, O-sensei used these kiai (my computer insists on "kiwi"
) in specific ways . . . do you see a connection???
Do any Koryu aficionados???
FWIW, (very little no doubt) I learned these in the context of Ken . . . (I learned almost everything that was "a thing"* in the context of Ken from my teacher!) [Back in the "old days" that was the the quintessential answer to any conundrum raised in the dojo, "Get your ken!" I kind of miss the "old days."
Yes, certain Kiai are supposed to have certain effects (different effects for different Kiai) on the nervous system. Certainly, under the same theory, that would work the other way as well, as a conditioning method.
OTOH, I'm not sure whether it's the specific syllables, the specific intent, or the specific delivery that produces the effect. Would nonsense syllables produce the same effect if you believed that they would? Or is the human body biologically "tuned" to certain auditory stimuli?
O-Sensei came from a certain background, with certain beliefs, so some things would affect him in different ways (I expect) from somebody raised in a different manner. Or maybe not - it would be interesting if we could really put it to the test on a blind group...