While De Palma handled all of the film making aspects of the work, he left it to Paul Williams to bring Phantom to life musically. And he did a great job, this piece, "Faust," has a beautiful piano theme that reflects the titular Phantom's plight as an artist within a corrupt industry. In fact, if you're looking into a more in-depth analysis for the film, check out this article that deals with its connections to the original Phantom of the Opera and the lengths that Williams went to in order to create a perfect musical setting for it: http://theampedblog.com/2014/01/fandom-opera-part-1/
Got any favorite musicals?
