I was scrumbling around in YouTube last night, too lazy to go to bed, when I came across this – the internet in its heavenly aspect:
Without YouTube, I might have gone to my grave without hearing this.
You can have the fun of doing the research, but you can start with Wikipedia: Trallalero
Here are some modern guys, doing a wonderful job of it – the first group, that is, up to 4:35. (The folks after that are earnest, but not up to the task.) Note the charismatic tenor who moves up to contralto at 2:40, and the little Danny DeVito type doing the mouth guitar.