Troy Baker, amateur opera singer and voice of the Joker in the upcoming Batman: Arkham Origins, gave a live demonstration of his vocal talents to New York Comic-Con attendees, and the results are ... well, skip to 2:12 in the above video and see for yourself.
The speech Baker reads in the video is from a one-shot Batman comic called "The Killing Joke," written by Alan Moore. In the comic, the monologue begins while Joker tortures Commissioner Gordon by chaining him to a carnival ride and forcing him to view pictures of his daughter Barbara Gordon (who at the time was also Batgirl), naked and paralyzed from a gunshot wound Joker inflicted on her. So yeah, it's kind of dark material.
If you watch before Baker gives his performance, you'll also see him describe the audition process. Apparently, Baker didn't even know who he was auditioning for until part-way through. "I remember that, walking into an audition and I think that, actually, it wasn't even Joker, it was a character whose name was just 'Jack,'" Baker said. It wasn't too long before Baker realized what he was reading, as well as the pressure he was under.
Baker also sings a little jingle at the end of the video, which we'll recite for you now: "Some say I am good. Some say I am evil, even more so than the Sith. But I suppose you'll all find out on October 25th."