The “woman singer” on “Kiss At the End of the Rainbow” was Katherine O’Hara. It’s not like she’s an unknown or anything. She was on SCTV, she was Eugene Levy’s slutty wife in Best in Show, she was the mom in Home Alone. Ring any bells?
I feel your pain, Diogenes. 
No one claimed it was. They were talking about the fourth song, A Kiss at the End of the Rainbow, from the mockumentary A Mighty Wind, sung by Mitch and Mickey (Eugene Levy and Catherine O’Hara). Mickey was playing a chorded zither, or Autoharp.