The term is "matting." We know it more today as "greenscreen."
My thoughts: They have done the microphones in front of a greenscreen, and then blend the two.
I learned the technique when we lived in California. As a family we would go to Universal Studios, NBC and CBS. You can learn a lot taking tours of everyday things. When I was in Jr. High I remembered what I learned there, and along with a couple friends we did this during a school project.
I don't know how they do it with digital, but with film it was a double exposure using a green filter on the camera. From time to time the person in front of the greenscreen would have green in their wardrobe, and then the other objects would show up where it wasn't supposed to.