Navigate to Style Your Video screen for your video.
On the lower bar, pick the audio Icon
3. Add a music bed background behind your narration:
Choose Audio samples by pressing on "Music" tab.
Use the slider to set up the audio balance between your voice and the background (recommended 95% "my voice"/ 5% Music).
4. If your audio in the preview is very low, you can use "Boost Audio" feature to boost the audio levels.
Please note: using "Boost audio" will be effective only if your recording does not have too much background noise.