We've all had it happen, or something like it. In my case, it was an Ultimate Frisbee highlight video I was editing with music and everything. For a number of clips I wanted the music to dominate up until a score, at which point I wanted the hooting and hollering to win out. The handy rubber band in iMovieHD always obliged. Not so in iMovie "the Rebirth". In your case there may have been the occasional car vrooming by or an unwelcome bout of flatulence by Uncle Ralph. Either way, just because the rubber band ran off doesn't mean your hopes did too.
Okay, being completely honest you can't fade audio, or "duck" audio, within a clip, but you can accomplish the same effect. Another caveat is that this How To does not cover all audio editing issues you will face. This is just about audio levels in general and you may come across issues with audio levels that this post does not address.
Here is how to fade audio in iMovie '09.
Video Killed the Radio Star
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!"
Well, if you are really picky about your audio levels, editing the audio of the sound track itself looks to be a much messier operation if you want to make the change for only the portion of the clip. For a song, from what I can tell it would involve actually importing the song three times and trimming each of the three copies of the song to the particular fade points, reducing the volume of the middle copy, and lining all three songs up in the "Arrange Music Tracks" window. Even then, I don't think the audio would fade, but just drop off then jump back up. The auto-ducking option suddenly makes me feel a lot less picky.
Of course, if you want to fade an entire song, you can select that song, open the "Audio Adjustments" window I mentioned earlier, and just reduce the volume on the volume slider.
What if I want to duck more than one clip?
You can prioritize the audio on as many clips as you like. To do this, you could use the old fashioned way and, while you have the audio adjustment window open, click on the each of the other video clips and check the same "Reduce volume of other tracks" button. Or, you could use the fancy-new-super-duper-mega-cool way and select the first clip you prioritized, press "Command-C" to copy the settings of that clip, and then select each of the other clips and press "Option-Command-U" to paste the audio adjustment to those clips. (The "audio paste" ability can also be accessed in the Edit menu under "Paste Adjustments".) Now that you have done that, you can feel totally ridiculous because the fancy-new-super-super-mega-cool way took the same amount of time and effort. Curse this new iMovie and its inability to select multiple clips at once!
(An Aside: Selecting video clips for adjusting audio has a slightly confusing UI-thing going on. Consider the following:
If I select a clip and open the "Audio Adjustments" window, I will get this. This looks normal.
If I then immediately click on another clip to edit its audio, I get this. This is confusing. For which clip am I editing the audio? The one on the right with the fainter yellow border.
If I had originally selected only part of a clip to edit its audio and then open the "Audio Adjustments" window, I get this. This is a little weird, but it at least tells me that I am editing audio for the entire clip.
This is behavior I don't expect to continue into future versions of iMovie because it will probably change as iMovie's audio editing abilities are improved.)
How do I make the video quieter than the song?
You might think that all you need to do is select the entire music track and choose the "Reduce volume of other tracks" checkbox. It doesn't work. That option is greyed out for music tracks. Lame.
Again, focus on the video clips, not the audio tracks. In this case, you want to select the video clip you want to be quieter than the music and, upon opening the "Audio Adjustment" window, reduce the volume on that clip by moving the volume slider to the desired amount. iMovie knows to fade the clip, so don't worry about a sudden volume change when your movie gets to that point.
If you want to do the same to multiple clips, you might want to use the "audio paste" feature I described above. It's not much of a time-saver, but will help you be more consistent. If instead you decided to drag the volume slider for each clip, it opens the opportunity to get different levels between clips.
Quick Tip: If you plan ahead, you can save your self a little monotony in changing the audio levels for a lot of clips. Before you begin adding video clips to your project, select the source video you are using in the Event Browser and make the audio adjustments you need. All of the clips you drag into your project from the source video will land in your project with the same audio adjustments.
How do I make these changes within a clip?
You can't. iMovie can only make audio changes to entire clips. This doesn't mean you can no longer get the same result. The trick is to create a new clip for the range of video you need to adjust. Odds are you want to make sure your audio adjustments cover a very specific range of audio. If you haven't already seen it, I covered how to make frame-precise edits in another post. Go read that if you need help making accurate video edits.
How to make frame-precise edits
The easiest way to make a new clip for a range of video is to use the "Split Clip" ability from the Edit menu.
This will take the range of video you selected and make three clips: the video before your selected range, the selected range itself, and the video after your selected range. If you make a mistake in splitting, just select the middle clip and choose "Join Clip" from the Edit menu.
Once you have made a new clip from the range of video you need to fade up or down, just select the new clip, open the "Audio Adjustments" window, and do what you need to do.
The rubber band won't bounce back
Looking over this How To, many people can complain that the rubber band was more elegant, intuitive, and accurate. They're right. For example, you can't extend an audio fade for over a second or two. Nor can you easily adjust the audio levels of multiple tracks all in the same window. In spite of these reasons, I don't think that the rubber band will make a comeback in iMovie. In its place, I hope this How To helps.