I found this helpful from an online posting. I think it will help you. Consider testing it?
Paste starts here (Thanks to the guy who wrote this):
Here is how to fade audio in iMovie '09.
Editing audio levels in iMovie '09 is very "video-centric." In iMovieHD, editing audio always involved going to the actual audio track and making changes. If you wanted a track to be quieter than another track, you reduced the volume on that track. With iMovie '09, you will have a much easier time by generally focusing on making audio changes to the video track. Put your song in place. Put your sound effects in place. Once they are in place, focus your audio level editing on the video tracks. This might seem counter-intuitive to you like it did to me, but it helps me focus on how the new iMovie works.
How do I make the song quieter than the video?
First it's important to understand how audio ducking works in iMovie '09. In the interest of simplicity, Apple automated the audio ducking process. Instead of having to manually fade an audio track in a sound or video clip, iMovie will subordinate one track to the other depending on the audio settings for a given clip. This is pretty reasonable, as long as you like the actual levels iMovie gives to the quieter track.
To auto-duck other audio tracks, select the clip you want to dominate and either hit the letter "A" or select the "Adjust Audio" button.
That will bring up this floating window:
Notice that I have already checked the box titled, "Reduce volume of other tracks." You, too, will need to check that box. Once checked, all other audio tracks that run during that video clip will fade to become quieter and then, when the clip is finished, will fade back up to full volume.
To this you may say, "Wow! That's great. No more fiddling around with the sound levels. All is as it should be." I suspect a lot of you are saying, "Boo! Bad iMovie! Who's in charge here? I am the director of this movie, not some algorithm!"
Source:
http://www.unlockingimovie.com/2009/01/how-to-fade-audio-within-clip.html