Before Jodie Foster was attached to this movie, the character of Erica was supposed to be a newspaper reporter. Foster changed her job to radio host because she thought that her being a print journalist didn't "didn't set a mood for the film and it wasn't as compelling in terms of the narrative" as being a radio reporter was. Foster also wanted to specify that Erica worked for the real news organization National Public Radio, but NPR declined to allow their name to be used in the movie because of the vigilante aspects of the plot.
In the scene where she receives a package and before the opens the box, you can clearly see and hear something moving on the floor in the room behind her (through the glass doors). It looks like a stand for a microphone or camera being pulled out of the view.
In closed Union cities like New York, radio talent would not mix their own audio (as during the call-in); this would be done by an audio engineer/production specialist.