We worked on something just like this years ago (over 10 years ago) at an engineering internship I had at an automotive supplier in Michigan. We had a whole mock-up for an auto show. We put directional microphones in a rear-view mirror and sent the signal through an amplifier to the car speakers. The biggest problem we had to overcome was feedback. It is really hard to amplify someones voice in such a small, enclosed space without horrendous feedback, and feedback at all kinds of audio frequencies. I remember we bought this off-the-shelf commercial feedback killer and mounted it under the dash just for the demo and it even had a hard time killing all the feedback.