"The song is called Ave Maria, it's considered to be a catholic hymn. Some of the singers are seen kneeling and holding prayer beads as they sing - the song itself is considered a prayer to the Virgin Mary, and the Notre Dame itself is dedicated to her."
Actually, the only thing that wasn't stolen at the Louvre's Apollo Gallery is the Regent Diamond, figure no one would take it because something to do with a curse.
I've also found that the two thieves were leaving for Algeria, thanks of a trace of DNA evidence recovered from a helmet and their past in burglary cases according to the police.