[Vol 1] Songs not on OST CD

I have noticed something while watching the DVD. As the title card for “Chapter 2: The Blood Splattered Bride” fades away, it opens on an aerial view of the bodies in the chapel, surrounded by forensics etc. and there seems to be a short, almost unnoticeable extract of (what sounds’ like) a Mexican song. No one has ever mentioned this song and it is not even featured in the credits at the end. Although it is small, i am still curious as to what it is