In iMovie 5 and earlier, while adding a voice track to a movie using the Record Voice feature, the recording stops after 5 minutes.
iMovie stops Record Voice after five minutes of continuous recording. This is an intentional limit to avoid wasting disk space if a user forgets to stop the recording manually, because the Record Voice feature uses about 10 MB of hard drive space per recorded minute.
To continue recording, click the Record Voice button again. If your previous recording stopped while you were speaking, you may need to crop the audio clip. See the iMovie online Help section "Working with Audio" for more information about cropping audio clips.