1. How to extract audio from movies with iLife
2. How to add enhanced audio track to iMovie?

After extracting audio from movies, you then proceed thusly. Set about your dirty work in Soundtrack or whatever audio application you care to use and do anything you like except change the duration of the audio file. So, play with the EQ, filter noise, or add an unhealthy amount of reverb, just don't cut or add anything to the file. (Because if you do, the file won't be in sync when you add it to your iMovie project.)

Then how to add the enhanced audio to the movie and have it synchronize with the movie?

When the file is to your liking, save it to the Desktop. Open your iMovie '09 project and drag the saved audio file from the Desktop to the beginning of the project in the Edit Project pane. That project pane should turn green and the name of your audio should appear in white at the very top of the Edit Project pane. Click the video track, choose Edit -> Mute Track, and play the video. You've muted the video's original audio track and now hear just the enhanced track you added. Everything should be in sync.

You can leave things just as they are or, if you like, you can delete the original audio track. To do so choose Edit -> Detach Audio. The video's soundtrack will appear as a purple bar beneath the video track. Click on that purple bar and press the Delete key. You've now left with just the audio track that you "sweetened" with an audio editor.