Audio over USB in Logic 9. M/S recognised but nothing happens

Hey! I tried to connect the M:S only per Audio in Logic 9 pro. When I went to the Environment it offered me to choose a M:S MIDI Interface (New/Instrument).
I then chose this Instrument in my Arrangement. I created another track (Audio 1/2) which I input, of course.
But no sound.
Anyone can help?
Thank you in advance