Is there internal microphone in the Apple TV 2nd Generation?

There is definitively no microphone in ATV- no reason to justify the addition of an analog audio Codec in this box. If it were the case, there would also be an analog output (likely on a 3.5mm stereo jack).

If video-chat was to be added, I would imagine that it would be through an accessory that would provide both video and audio capture, better located than ATV that may be hidden in a cabinet...

This said, most webcams use the full USB power, or 500mA/5V, which is half the total power available... Not sure there is enough overhead for this...