public address system 
noun
A system of microphones, amplifiers, and loudspeakers used to amplify speech or music in a large building or at an outdoor gathering.They will now be supported by the new public address system - with loud speakers placed strategically on cameras throughout the city centre....
- For example, if they go to a meeting, they should strongly suggest that the speaker use a public address system.
- The periodical announcement over the public address system about the services available brings more people here.