In the Hunger Games arena, the impression is that everything is recorded and televised. As well as technology enabling them to insert new features (fire, mutts, parachutes, etc.) into the arena wherever and whenever they like, the Gamemakers have cameras everywhere which are broadcasting the events of the Games to the whole of Panem. These cameras are hidden in strategic locations so that pretty much anything can be picked up and broadcast.
How about sound? Do the Gamemakers also have extremely sensitive microphones dotted about the arena so that any word spoken anywhere by anyone is also broadcast? That seems implausible - as far as I know (though I'm no expert), a mic usually needs to be pretty close to someone to be able to transmit their words clearly without any extra baggage.
The obvious answer is that the tribute costumes are equipped with tiny microphones so that everyone's voice can be picked up and transmitted on television. Is there any confirming evidence that this is the case?