Apple Final Cut Pro 7 - Manual (Page 128)

128 Chapter 6 Working with Audio
Decrease the audio level over time using keyframes:
Position the playhead where you want the audio level to begin to decrease,
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somewhere near the end of the clip.
Position the playhead here.
Click the Level Keyframe button to add a keyframe to the clip at the position of the 2
playhead.
Press End to position the playhead at the end of the clip, where you want the audio to
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reach a minimum level.
Click the Level Keyframe button to add a keyframe to the clip.
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