Audacity: Make Mono Track into Stereo (Plus Stereo Back to Mono)
Channel: TanUv90
Date: 28/04/2012

Creating a Stereo track from a Mono track is pretty easy. Although it's not "True" stereo, because it is basically a duplicate of the Mono track in both channels, it is useful when you are using an Effect like Vocoder which requires a stereo track. Or you want just a stereo track to edit.
►Mono to Stereo:
Open and Select your Mono track in Audacity
Click Edit then "Duplicate" (Ctrl+D)
Click the Track Drop-Down Menu of the 1st track and select "Left"
Click the Track Drop-Down Menu of the 2bd track and select "Right"
Now click the Track Drop-Down Menu of the 1st and select "Make Stereo Track"
Now when you Export the file, it will be saved as stereo

►Stereo to Mono:
To make a Stereo track Mono, simply click the Track Drop-Down Menu and select "Split to Mono". When you Export the clip, Audacity will warn you that your tracks the will be mixed into a single mono channel. That's what you want so say OK and you're all set.

Download Audacity: http://audacity.sourceforge.net/download/