I think there's a little a '51 can do in such application - sure it could have its role but rather with low priority. All that, either you transfer as analog signal or convert to digital using a DSP. A '51 may take care of driving the camera, shutting off or tuning volume of the mic and speaker, receiving commands from PC etc. But the data signals (Audio/video) wouldn't even have a reason to pass through the '51... So you wouldn't do this -on- a 51 device...
Just enter some major manufacturers websites (Maxim, TI, Analog Devices) and search their part catalogs to see what is available and to get general idea what you can achieve.