In the video editor, tap the Audio tab.
Use your own audio
Tap Add a voice-over or music, then choose an audio file from your library, or tap Upload new to add an MP3, WAV or M4A file up to 25 MB.
Generate a voice-over
Voice-overs come with the Week, Creator and Pro plans.
Tap Generate voice-over.
Pick a Voice and play its preview.
Write or edit the Script. The character count and credits show below it.
Tap Generate voice-over.
Once a voice-over is added, tap Change, Generate new or Remove to swap it.
Fit the video to the voice
When the voice-over and scenes are different lengths, tap Fit scenes to it to stretch or shorten scenes so the video matches. If the voice runs longer than your scenes can cover, add more scenes.
Volume
Set the Voice-over level, and the Clip sound level when the video has clips. When you add a voice-over, clip sound is lowered so the voice sits on top.