A song says which console it sounds like with chip, and that decides its voices. The NES has two squares, a triangle, noise and dpcm, a voice of samples; the SNES ('snes') has eight voices of samples and an echo; the Genesis ('genesis') has six FM voices plus the squares and noise of its second chip.
Every voice here plays with one of the engine's instruments, asked for by name (lead, strings, fm-lead, slap-bass…), so the song defines none of its own. Each voice also has a pan, between left (-1) and right (1), and song.pan can be tweened: P carries the arpeggio across to the other side.
Space to play, 1 to 3 for the console, P to move the arpeggio.