“I recently heard some non-Jewish people singing the melody used
in a Simcha Leiner song – don’t know the title, but it includes
the lyrics “sasson v’simcha.” Can anyone name the original song?”
It’s actually better not to know these things. If you know the original words, and if they are assur, then it becomes assur to listen to the Jewish song if you end up thinking about the original words. So says R’ Matisyahu Salomon, Shlita, in one of his books. Otherwise, it’s not a problem.