Apple Final Cut Express 4 - Manual (Page 635)

Chapter 42 Mixing Audio in the Timeline and Viewer 635
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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.