Found that medieval Spanish song (in case anyone cares)
NOT on the OST ... and not mentioned by anyone here either AFAMECS.
It's a traditional Spanish song called "¡Ay luna que reluces!".
Here's a version of it, alas, not the version used in the movie:
http://www.youtube.com/watch?v=ntXeUTRxaHo
Though identified elsewhere, I thought I'd copy it over here for anyone interested.