When you export a combined video from Lookback you should be able to hear sound on the recording.  

If you can't hear sound, it could be because of the media player you are using.

Please try opening the video with Quicktime or VLC (http://www.videolan.org/) and check if you can hear the sound.

Note: The download/export feature is only available under the Pro (Legacy), Enterprise, Freelance, Team, and Insights Hub Plans