Well, one of my all time favorite groups was The Persuasions, and later Jerry Lawson and Talk of The Town (Jerry was the lead singer in The Persuasions). Here is one from their tribute album to Frank Zappa. Also of note is their tribute to The Grateful Dead. Oh, hell. Every thing they ever recorded was outstanding. But you will like this A Capella music or hate it. I have not found anyone that was just so-so about it.