AFAIK, the mike input is not a normal stereo input. It is a mutant thing designed for use with the "Plaintalk" pre-amped mike. And it's mono. I'm sure someone will correct me, but I believe that you will need a separate card to get stereo audio in.

For some possibly relevant info, see http://www.troi.com/tips/plain.html. The wiring data there also seems to support my recollection.