How to create a split edit effect with iMovie 11

1. The Precision Editor

Find a clip in your project whose audio you'd like to extend and mouse over its thumbnail. You'll see a button appear lower left of it. Click on it and select the first option: Precision Editor. The lower part of the interface is replaced with a more zoomed-in version of your clips and their edit points.
2. See waveforms

By default, you'll only see the video clip. Just like the project section, you can zoom in or out of it with the slider, bottom-right of the section. If you want to see the clip's audio track, you need to click on the Show Audio Waveform button, top-right of the interface.
3. Create the split edit

Move the cursor to the edit point's blue line and make sure it's over the audio waveform, not the video. Now drag to the right to extend the audio but leave the video exactly where it was. You can do the same for the clip beneath it so you don't hear two audio tracks at the same time.