Apple Final Cut Pro 7 - Manual (Page 942)

5 Drag the Pan slider all the way to the right, so that the sound ends playing out of the
right speaker. Because you’ve already set a keyframe for this clip, dragging the Pan slider
at another point in the clip automatically produces a new keyframe.
When you play back the clip, you’ll hear the car sound move from left to right.
942 Chapter 57 Mixing Audio in the Timeline and Viewer