Note: These instructions are for iTunes version 8. Other versions may put the encoding selection options in slightly different places, but the concept is basically the same.

1. Export your audio files
2. Open iTunes
3. From the iTunes menu,  select Preferences.
4. Select the General tab and click the Import Settings button.
5. From the Import Using section, select MP3 encoder.
6. From the Settings section, choose Custom and a new dialog  will appear.
7. Set the Stereo Bit Rate to 128k and set the Channel to Mono.
8. Click OK on the Custom box and on the Preferences tab.
9. Now go to your Playlist and import your audio file (you can simply drag files into the playlist).
10. Select the track you want to convert.
11. Pull down the Advanced menu and choose Create MP3 Version.
12. When encoding is complete, the file will appear as an MP3 in the Music Library with the same name as your original file. You can figure out which format the file is in by by selecting a track and hitting Cmd-I.

Click "Import Settings" from iTunes General Preferences tab.

Set the MP3 encoding preferences to 128k mono.