Splatoon 2 includes a hidden Rhythm Minigame/Sound Test mode accessible in Inkopolis Plaza. It includes 42 playable songs, each completable on Normal and Hard difficulty.
Aside from the gold and silver squid badges displayed after beating an individual track, are there any rewards for completing every song?
Polygon is reporting that there are no rewards:
Going through each of these tracks is a pleasure not because of any rigorous gameplay or rad rewards — there are none of the latter