The engine has songs of its own: old pieces that belong to everyone (Beethoven, Grieg, Bach, Joplin, folk songs…) arranged for the NES, the SNES and the Genesis. BUILTIN.MUSIC.SONGS lists them, with their name, composer, year and console.
Each one is asked for by its key, useLoadSong({ src: 'nacatamalon:ode-to-joy' }), and only downloaded then. From there it is a song like any other: setTempo and setTranspose change it while it plays.
Up and down to choose, space to play, left and right for the tempo, - and = for the key.