This is very similar to the Colosseum in Rome. It was actually built first and is significantly smaller. According to my tour guide certain musical artists such as sting and coldplay had performed there. It is made so that you don't need microphones (because they obviously didn't have those back when this was built!) and when you speak everyone in the audience can hear you but the sound does not escape the top. So if you stand right outside of it during a show without microphones you can't hear anything.