- Audio tab → Music
(Pro).
- Upload a music file
(MP3, WAV, OGG, M4A, AAC, FLAC or WebM).
- Set its volume.
For voice-overs, keep music very low.
- Export.
Loudness normalisation keeps the final mix at a consistent level.
Use music you have the rights to. Royalty-free libraries and YouTube's Audio Library are good sources.