I haven't read all the posts, or the other threads linked to, that thoroughly, but I'll jump in with both feet.

One theory I heard, advanced by Dr. David C. Fowler, (University of Washington English professor and author of several books on early English Literature, including one on balladry) when I was taking a course in "The Popular Ballad" from him, was that the nonsense syllables in many older songs and ballads may have originally been in Latin and possibly had some religious reference, and subsequent singers, not understanding the language, "folk processed" them into nonsense syllables. Or may have been an attempt by non-Latin speakers to imitate Latin choruses they heard in church.

Same with words originally in other languages, Gaelic and derivative languages certainly sound plausible.

Some of it, I'm don't doubt, is pure nonsense "fill," a la mouth music or scat-singing.

I don't think any one theory applies to all nonsense choruses. Maybe some each and a bit of all.

Don Firth

P. S. Then, of course, there are some choruses or refrains that are not nonsense syllables, but don't really relate to anything in the song or ballad. For example:

She leaned her back against a thorn,
Fine flowers in the valley,
And there her little babe was born,
And the green grass it grows rarely.

(Version of The Cruel Mother, Child #20)

It does serve the function of filling out the song, adding a bit of suspense, and generally enhancing the somewhat eerie supernatural nature of the song.