Apple Final Cut Express HD - Manual (Page 621)

Chapter 44 Mixing Audio in the Timeline and Viewer 621
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4 Now, move the playhead to a position after the car sound effect has finished playing.
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.