I was trying to describe the lyrics of a song today and stumbled when I reached a section with a bunch of "la la la" bits. I suppose I could have well just termed them as la-la-la bits, but seeing as to how I apparently have all the time in the world (not my words), I decided to see if there was something a bit more definitive that I could use. A wittle googling later, I chanced upon the answer.
They are called non-lexical vocables and, besides "la la la" and "la dee da", include a range of other variants. Jazzy scat singing, beatboxing, A capella, and presumably stuff that you can hear in rhymes and fairy tales (fee fi fo fum?) all fall under this rubric.
So does this.