Apple Final Cut Express 4 - Manual (Page 630)

630 Part VIII Audio Mixing
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.
Finally, you’ll want to move the outside pair of each group of four keyframes outward a
bit, so the volume of the music doesn’t change too abruptly and startle the audience.
Less steep slopes between keyframes result in more gradual fades from one audio level
to the next.
Example: Setting Subframe Audio Level Keyframes
to Eliminate Clicks
Sometimes, when you find the perfect edit point for cutting a clip into your sequence,
you’ll notice a pop or click in the audio. This happens when you make a cut on an
awkward sample that just happens to occur at a frame boundary.
You can eliminate pops and clicks by setting keyframes for your audio levels to within
1/100 of a frame. Usually, changing an audio edit point by just a few hundredths of a
frame eliminates the clicking.
Lowered level
Adjust the slope of
the level change.