Apple Final Cut Pro 7 - Manual (Page 129)

Chapter 6 Working with Audio 129
Drag the Level slider to the left to decrease the audio level all the way. 5
Drag the Level slider to
decrease the audio level.
Play the clip to hear how the audio fades over time. 6
Experiment with dragging the keyframes and audio segments up and down to 7
familiarize yourself with changing the audio level using keyframes.