I can't find anything like that in the scripts, sorry. I googled them all. The closest I can find is "My Unicorn", the one where Matthew Perry (from Friends) plays the long lost son of the guy needing a kidney transplant. In it, JD sings the "I found his son I found his son I found his son…." song, and then Perry jumps out and says "It's me!"

in what episode dous JD narirate "ok if you dont go ill sing (..in song…) for the rest of the day" and than goes. if it helps i think it was on january 31 on reruns on comidy central, and also what was the song.
Thanks