If you're piecing together a documentary or any video with a mix of talking heads and images, adding a soundtrack is very common. However, you'll often find that simply throwing a song into the project will overwhelm your speakers. You might think, "Ok, let me just lower the volume of my music track." You would be correct, however, this can lead to the music being too quiet when there isn't anyone talking (that is, you're showing some other visual image between interview clips). FCPX makes it easy to select a range within a clip and adjust its volume by automatically adding keyframes. This process is sometimes referred to as ducking your audio .
In the timeline shown in the following screenshot, we have an interview clip at an appropriate level, but we've also added a music track. The music track is at an appropriate level when played over b-roll or other silent images, but unfortunately overwhelms our speaker.
