What is the song playing in the ball scene of romeo and Juliet?

The song is called "O Verona" and was composed by Baz Luhrman and Craig Armstrong and is available on William Shakespeare's Romeo + Juliet: Music from the Motion Picture Volume 2. Unfortunately, it cannot be downloaded from iTunes.

You're probably relating it to the classical choral composition "O Fortuna" by Carl Orff which became unable to be used in films, though I'm not sure why, so now in trailers people will use the O Verona composition instead. They wanted to use O Fortuna but were denied and so used it to inspire their own composition.