"Once Upon a Time in Mexico" question...

[Mockery]

The soundtrack for this movie is a damned good one and I intend to get it soon, but I heard that the song that they play during the intro isn't on it. This is a damned shame to me because it's easily some of the best music in the film.

Anyway, I'm trying to find out the name of this song. If you don't know what song I'm speaking of, it's the one that starts playing at the beginning when the old man gives Banderas the acoustic guitar and then he starts walking through the streets while playing it. Any help finding what this song is called would be greatly appreciated.

Thanks.