Apple Final Cut Pro 7 - Manual (Page 954)

In the next example, all three audio tracks already have audio edited into 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.
Before
The newly recorded
audio clip After
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.
954 Chapter 58 Using the Voice Over Tool