Apple Final Cut Pro 7 - Manual (Page 936)

There are long pauses between the narrator’s lines, during which you want the music to
be the dominant audio track. So you set the overall level of your music to -4 dB, because
that’s the level at which the audio sounds best between the narrator’s lines. When the
narrator speaks, however, you want the level of the music to drop so it doesn’t compete
with the narrator for attention.
If you hold down the Option key (while the Selection tool is selected) and click the audio
level overlay of the music clip with the Pen tool, you can place groups of four keyframes
at each place where a line is spoken by the narrator.
Keyframes set
Then, releasing the Option key, you can drag the area in the middle of each group of four
keyframes down, to lower the level of the music while the narrator speaks.
Lowered level
936 Chapter 57 Mixing Audio in the Timeline and Viewer