How can I broadcast what I hear in my headset through my speaker system?

From the Lecture, Practice, or Group mode button, Press the Volume tab from the row of tabs at the bottom of the screen.
Press the "Room On" button. This button routes any signal in teacher headset to the external speakers set up in the classroom. This feature is useful if you want to have student's remove headsets and listen to a musical piece, or if you have a situation where there are some people present in the room who are not on a GEC3 unit (for example: concert for parents).