The [LeveManager] prefab comes by default with a Level Music Manager component attached. If you don't want music in your level, you can remove it by right click it and choosing Remove Component.
Music in Orchid Rain levels come in pairs. There's a normal version and an "X" version of each level's music. The normal version will play always by default. The X version will play on an arbitrary point in a Mission, usually accentuating a moment of intense action.
You can assign a soundtrack from the game to your level, by choosing a value in the Music Set dropdown, but you can also add your own music files. If you leave Override Music and Override X Music as None, the game's vanilla music will play. Just drag and drop some audio files in those fields to give your level its own soundtrack.