I have had this problem plaguing my video's lately…I have an external mic plugged into my video camera and when I record the audio it comes out as a mono vs. stereo channel.  I know that there are some very sophisticated ways that I can handle it in pre-processing but I had already recorded it as a mono track.

I came up with a quick way to emulate a stereo audio track in iMovie and wanted to share the how-to.  

  1. Basically you export the audio via aiff
  2. Import it into audacity
  3. Duplicate the mono channel into two stereo channels
  4. Assign the left and right channels
  5. Export and then import into iTunes
  6. Detach audio from the live clip in iMovie
  7. Attach the new "stereo" clip to your movie

Having a stereo track makes a huge difference in the perceived quality of a movie.