This page has info on how to do some tasks in Audacity.
To combine a stero project (has left and right tracks) to a mono track.
- Load the project.
- Select the "Split Tracks" from the Project Menu (the drop down menu in the project window).
- For each track in the Project Menu select the 'Mono" option.
- Then highlight the tracks you want to combine into a single track.
- From the top Menu bar select Tracks -> Mix and Render