[Update: According to a source I trust, the bug has not been fixed. I haven't been able to replicate it in a project where I originally saw the bug, however.]
It appears that Apple fixed an unusual but maddening bug in iMovie '09 8.0.3. As I detailed in "iMovie '09 Tip: Speed Clips 2,000%," the Speed control in the Clip Adjustments inspector isn't limited to the preset tickmarks on the slider (which let you change a clip's speed to anywhere from 12.5% to 800%). By entering a value in the slider's text field, you can set the clip's speed to any number between 5% and 2000%.
Unfortunately, a bug in iMovie messed up audio in the project when exporting the movie to iDVD or movie files; it could affect audio in any clip within the movie, not just the clip that had been sped up or slowed down.