Theatre of Epidaurus
The Theatre of Epidaurus, and yes you can sit in the very back row of the theatre and clearly hear the conversations on the stage. The theatre was built in the 4th Century BCE, about 2400 years ago, and is part of the massive healing sanctuary of Epidaurus, the best in the classical world.
The theatre is still in use, and has seating for about 14000. Tickets are available here.