Apple Final Cut Express 4 - Manual (Page 477)

Chapter 32 Audio Editing Basics 477
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To create a section of room tone from a short pause:
1 Find the longest pause you can in the dialogue clip with the gap you need to fill, then
copy the section that contains the pause. If you’re in the Timeline, you can use the
Range Selection tool.
2 Create a new sequence, name it “Room Tone,” and paste the audio pause into it twice.
3 Select the clip containing the second pause, then choose Modify > Speed.
4 In the Speed dialog, click the Reverse checkbox to select it.
The long pause in your
clip is selected.
Paste the pause section
you just copied twice
into a new sequence.