1. Save your level.
2. Go to the "Sounds" tab (you should be there already, or you can press 1 to go there).
3. Click on the timeline grid to create an event. A menu will appear, click "Play Song".
4. Click on the folder button and look for the music file you want to use for the level.
5. That's it! Rewind and click the Play button.
You can also follow this tutorial: https://rhythm-doctor.gitbook.io/level-editor/adding-a-song/song-bpm-and-audio-o...