Videoscribe does not include audio editing options. It would probably be best to use any audio recording/editing software to create your voiceover, then save it as an mp3 and import it into videoscribe.
Some people record the voiceover first, then build their scribe. Other people play the videoscribe preview while they record their voiceover in another program.
Or you can just record the voiceover in videoscribe as a last resort.