Hi Matt - the following extract from iMovie Help should get you back on track:
From iMovie '11 Help ( http://help.apple.com/imovie/#mov3b0fbc0a )
To pin background music to video
In the Project browser, move the pointer along the upper-left edge of the background music well until it turns into a hand, and then drag the top-left corner of the background music well slowly to the right. A pin icon appears in the corner of the background music well, and the background color changes from green to purple.
Continue dragging the pin until it’s positioned above the video frame where you want the music to begin playing.The background music well turns to purple, indicating it’s been pinned to a video frame.
After you pin music to a video frame, unless you “unpin” it, the music stays with that frame even if you rearrange the order of your video clips.
To unpin background music
If you’ve pinned background music to a video frame and now you want to rearrange the background music clips in your movie, you can unpin it.
In the Project browser, click to select the background music well (it becomes outlined in yellow), and then choose Clip > Unpin Music Track.
The Clip menu appears in a light gray bar across the top of your computer screen.
Here's an alternative method for adding music to your video (which I use in preference to the back ground method) -
Rather than dragging the music to the grey background area, drag it directly on top of a video clip, close to where you want it to start. When you drop the music, a narrow green strip (bar/track) will appear below the video thumbnails. You can slide this back and forth in order to get it to start in the exact position. You can also drag the ends in to shorten or trim the music track. Or drag them out again to lengthen the track. You can also split the audio and show the audio waveforms, adjust volume within a track and so forth. Check iMovie Help for more details.
John
Message was edited by: John Cogdell