Shoo-bop-a-loo-bop. Yes, I heard the Pictish theory before; the lost tongue surviving only in choral remnants. And there certainly ARE songs in which the original language of the song survives in a corrupted form in the chorus. It happens quite a bit with the songs of immigrants in N America. Cuilionn mentions a variant of "si£l a r£in" (in the database as Shule Aroon, Buttermilk Hill, Shule Agra/Johnny has gone for a soldier; and also in an earlier thread in Irish); another example from Irish would be versions of an Drim¡onn D¡lis collected in Canada.
But I would like to offer another theory about the origin of meaningless syllables. In many cases they have no particular function except to sound funny and be fun to sing together. But the custom may have risen out of vocables that were used as mnemonics to help memorise tunes.
Some of you will know about songs that pipers used to learn tunes. As I understand it, the songs were similar to singing a melody with the words "doh, re, mi, fa," etc. fitting exactly which note was played. But the system was more complicated than that because there were also syllables to represent the time the note was held and some of the fingered ornamentation. Thus a piper could practise without an instrument, moving their fingers as the sound symbols of the song indicated.
Scottish waulking songs have characteristic choral lines such as "o hi h-oireann o". Everyone waulking the tweed would sing these choruses together as they worked, while individuals would sing the verses. The verses can be adapted and improvised, but any particular melody has very specific vocables and they are supposed to be strictly adhered to. Given a line of the chorus, those in the know will be able to give you the entire tune. John Campbell and Frances Collinson did some analysis of how particular sounds(for instance the length of vowels)give the rhythm of the tune. I don't know what further study has been done on the topic.
Has anyone got a theory about why Scots Gaels "hi ho", Irish singers "diddle da dee", Jazz goes "beebop", etc - the preponderence of a particular initial syllable in the lilting and vocatives of different cultures or musical styles?