The music for a level, made in the Data panel as Music: its named tracks, each with an optional intro and layers, the rules for changing from one track to another, the change used when no rule matches, and a playlist to rotate through under them. A level's Music names it, and a script asks for a track by name.
Made in Assets with Create → New data asset → Music, in the folder last clicked in the tree, or the folder of the file last clicked, which the path bar names: type its name and press Create. The .data file opens in the Inspector, which draws it in place of the selection: that is the Data panel. Double-click the file in Assets to open it again, and close it with the ✕ before its name, which goes back to the node inspector. A level’s Music, in the Levels panel, names it, with the Track it opens on; a script’s play_music asks it for a track by name, and set_music_layer turns a track’s layer up and down.