By default, Ardour will export all audio in the range or session being exported. What it actually exports is all audio routed through the master output bus. You can see the list of tracks to export on the right side of the "Export" window. If you click the 'Specific Tracks' button, you will be able to choose from a list of all the tracks and busses in a session. Choosing specific tracks only makes sense if you do not want to export the master bus' output, so you should probably de-select that first.