It is not terribly obvious in Audacity as to how to convert a stereo file into mono, but it is easy. Open the stereo file. It will show up as two tracks that are tied together. On the top left of the first track, you should see a little “x”. Just to the right of that, you should see your file name, with a little drop-down arrow beside it. Click on the arrow, and then select “Split Stereo Track”. This will separate the two tracks. Click the drop down arrow again, and select “Mono”. Now, you will need to click the drop-down arrow for the second track, and select “Mono” for it as well. Finally, go to the main “File” menu of Audacity, and click “Export”. Here, after filling in metadata such as album, date, etc., you will be given the opportunity to change the file name, choose a destination folder, and what type of audio format to convert to. If all goes as planned, you should see a warning that the two tracks will be mixed down to one mono track, and when you OK it, Audacity will make the conversion and save the file.
Convert Stereo to Mono in Audacity