1. Select all the tracks (CTL-A, IIRC) whiile in the tracks portion of the Track View (not in the bus portion)
2. File-Export
3. In that dialog chose Tracks where it probably says "Entire Mix" (it's a drop down list on the left side of the dialog).
4. In the middle of the dialog there another drop down list that may say "Stereo" change that to "Follow Source"