At the moment, we do not offer the ability to create or utilize voiceovers.
Some of our users get around this by recording a voice-over in another application such as Audacity and then upload it to Biteable. If you do this, for better syncing, I suggest creating the video first and then record the voice-over.
Our product team is aware of the request for voiceovers and are always researching its possibility. Stay tuned!