Apple Final Cut Pro 7 - Manual (Page 131)

Chapter 6 Working with Audio 131
Press the Right Arrow key once to move the playhead forward one frame, then click 3
the Level Keyframe button to add a keyframe. Position the playhead at the end of the
sound and click the Level Keyframe button to add a keyframe at that point.
Place a keyframe
at the end of the sound.
Press the Right Arrow key once to move the playhead forward one frame, then click the 4
Level Keyframe button to add a keyframe.