If the original user had exported his song using the "Export song as Audio File" window, then it is possible he had both the "One file per track" checkbox and the "Include mix" checkbox checked. That would also include one audio file with all of the instruments and that is likely what is showing in the Audio track window.

If you want to remove it, then click on the track in the Mixer and go to Track Actions | Erase Track.


Best,
Chantelle