Problem

Sometimes you may wish to add, edit, or change the audio part of the video, in iMovie, and cannot figure out how to extract the audio.

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Solutions

iMovie 2 and 3

1. In iMovie, Select the clip which contains the audio you want to extract, then select "advance" -> "extract audio". An audio clip will be created below the video clip.
2. Select the audio clip, then select "File" -> "Show Info". remember the name of the media file.
3. Go to Finder, open your iMovie project media folder, look for the media file.
4. Open Garageband (if you don't like Garageband, you can use Sound Studio,Peak DV, or any other sound editing tool), then drag the media file from Finder to Garageband, a new track with the audio you want will be created in Garageband.
5. After you have done at Garageband, export the audio to iTunes or export an AIFF file back to disk
6. Click on the Audio button in iMovie, then drag the audio file from iTunes Library to the audio track or drag the AIFF file created

iMovie 4 and 5

1. In iMovie select "File" -> "Share"
2. Select Quicktime -> Expert Settings
3. Check or Uncheck the "Share Selected Clips" option. (If you want the whole movie deselect the checkbox)
4. Select "Sound to AIFF" / No Compression / 48 kHz / 16 bit / Stereo
   
5. Export out the AIFF file noting its location
6. Alter the audio with your program of choice. (Besides Garage Band, many use Sound Studio or Peak DV)

Here is a Quicktime Movie of this procedure

Note: If you have a new Mac you may already have Sound Studio in your Application or Utilities folder