Although, much like in our world, vowel harmony comes in many different forms in the Bryatesle-Dairwueh-Sargaĺk-Ćwarmin-Ŋʒädär world, speakers of vowel harmony languages seem to be aware of it as a typological trait that is missing from some languages but present in some. In fact, all the tribes that speak vowel-harmony languages share a certain addendum to the just-so stories of why there are many languages.
The usual narratives throughout the continent do not per se attribute any subjective qualities to the languages themselves – the attributes generally refer to each language's suitability for being spoken in its environment, often using pretty bad reasoning about it.
Often in these stories, the gods find that the world is less than it could be – mankind is organized, but badly. They find that the language that all mankind speaks is not good enough for the different needs the tribes have. Sometimes, a trickster god enters into the picture here, making the different languages be unintelligible - when this bit is present in the narrative, the gods usually planned for everyone to understand every language, but also for everyone to speak only the one relevant to their environment.
Now, speakers of languages with vowel harmony often add a little side plot to these narratives: the theme then being that some tribes were granted an aesthetical advantage over others: vowel harmony. Usually, the other groups are described as incapable of controlling some of their urges, and they thereby fail to live up to agreements they've done with the gods (or God) in exchange for getting their linguistic situation improved.
As a punishment, "the beautiful sound of words", vowel harmony, was taken from them.