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.
The masquerade ball was held in Act 1, Scene 5 of William Shakespeare's play "Romeo and Juliet."
Tybalt recognizes Romeo's voice at the ball in Act I scene 5 of the play Romeo and Juliet. He says, "This, by his voice, should be a Montague." He cannot see Romeo's face because he is wearing a mask.
In Act I, Scene V of Romeo and Juliet, Romeo and his friends crash the Capulet ball. Romeo meets Juliet and they fall in love at first sight. Tybalt recognizes Romeo and wants to fight him, but Lord Capulet stops him, not wanting trouble at the ball.
Capulet's ball
Juliet's garden to listen to her.
at capulet's ball/feast.
Romeo had hoped to see Rosaline at the ball. Rosaline was a girl that Romeo liked before he meet Juliet. Except Rosaline doesn't love Romeo back
Yes, Mercutio was at the ball in Romeo and Juliet. He is a close friend of Romeo and attends the Capulet's party with him.
TERRIBLE
because Romeo is from the Montague clan and Juliet is a Capulet. And the ball will take place in the Capulet's lair.
It was a Sunday, late in July. Nobody knows the exact date or the year--there is no clue as to these things in the text.
The impression Juliet has made on Romeo is what we call "love at first sight". When she first sees Romeo at the Capulets dinner/ball, she thinks of him as the most handsome man she has ever laid eyes on. Also, she is talking to herself on her balcony in scene 2 of act 2 about how Romeo should be hers if it wasn't for his last name, Montague.