Scenario: recent media coverage has gotten you, for the third or fourth time, to try to develop a better appreciation of The Beatles, but the stereo mix of Magical Mystery Tour practically makes you dizzy, with its way of putting all the vocals in one ear and all the instruments in the other. You might want to create a mono mix which, some argue, is the proper way to listen to the band, anyway.
Solution: use iTunes’s built-in encoders to turn the stereo copy you already have ripped into a mono mix.
First, open iTunes’s Preferences and click on “Import Settings…”
Pick either mp3 or AAC and select “Custom…” from the “Settings” menu.
Change “Channels” to “Mono”.
Click “Okay”, select the songs in iTunes, and choose “Create AAC Version” (or MP3) from the “Advanced” menu.
Make sure you change the settings back to Auto or Stereo afterward.