A couple things may limit your video from playing audio within the app.
Timelapse Videos Have No Sound
If you've implemented timelapse, using Speed, audio is off by default. Since the app is accelerating the speed at which your video plays, it wouldn't sound good if we accelerated the audio too.
Turn Ring/Silent Off
When enabled, your phone will mute sounds and alerts. Disable this and play the video again to hear its audio.
Turn The Volume Up
Your phone's audio setting might be too low. Try increasing its audio and play the video again.