I have a solution that works perfectly!
Executive Summary:
DELETE ANY GREEN CLIPS WHICH HAVE YELLOW TRIANGLES!!!!!
Full Details:
I was working on a tiny project, maybe 14 minutes about violence on television, and I wanted to add background music. So I added an aiff file at the start, but NOT as a background track. Then I did some desktop cleanup and I moved that file into a folder for my film's resources. I didn't think it would be a problem since I alreay added it to iMovie. I figured iMovie worked like iTunes in that it copied it's media to it's own folder!
WRONG!
Suddenly the background sound didn't work anymore. I looked at my project, and now there was a tiny little yellow triangle only at the front of the green audio clip. None of the clips that reference the moved file worked anymore! Pathetic of Apple, really! I double clicked it, there is NO WAY to pick the new location of that clip. You have to delete it from the project and make all the adjustments again. So i tried adding it again, however this time, it was completely silent. I figured maybe iMovie doesn't support properly AIFF files made by Audacity. Then I tested other portions and noticed that many of my detauched audio clips weren't playing either.
This ****** me right off. iMovie '11 has a TON of bugs if you've used it fully. It is absolutely pathetic of Apple to have released it!!! 😟 So I read on here that I can use a cutaway by dragging a video on top of another video, so I used the clip trimmer to extend a cut clip to cover the additional minute that was detached. This was problematic since you can only extend a clip by 10 seconds at a time. Another stupid crippled feature by the geniuses at Apple! But I am immortal, so I wasted my time. And I used cutaway without detaching audio, and it worked. Then I tried adding a crossfade, and discovered this was another crippled feature! You can't add a transition onto a cutaway. This absolutely angered me.
I looked throughout my entire project. And I found at the very end another broken green clip with a yellow rectangle from the background track I used. I used it for both opening and closing! I deleted it!
And ta-da! All my detached audio clips started working properly again!!!! Everything is now okay!
I know it's a year late, but I hope it helps others. Since Apple clearly has NO PLANS to fix iMovie nor add features which we already paid for in '06!! Really pathetic that a company can remove a room from your house without penalties!!