Why does trivia state that the song is unknown?
"Las Mananitas" is an adaptable song for birthdays, weddings and other events.
Here's a video of it:
http://www.youtube.com/watch?v=BL_cDdLwV9c
The song is notably performed in "Two Mules for Sister Sara".
Next time I see the film, I'll have to listen to the other music performed. I would add this to the information, but my previous attempts at providing information have failed though I was positive of what I submitted. Oh well... I did try.
*** The trouble with reality is there is no background music. ***