Apple Final Cut Express 4 - Manual (Page 647)

Chapter 43 Using the Voice Over Tool 647
VIII
In the next example, all three audio tracks already have audio edited onto them, and
the a2 Source control is connected to track A3. After using the Voice Over tool, a new
track A4 is created, and the new audio clip is placed there.
If another audio clip is already present in the audio track below the track connected to
the a2 Source control, a new audio track is inserted below this track. All previously
existing audio tracks below this are moved down to accommodate the new audio track.
Before
The newly recorded
audio clip
After