A secret bonus song was added to the game, and it was so secret I never heard about it until this news post that said it was added to the soundtrack.

The comments by a dev kinda explained how to get it, but it wasn't clear about it. Later on, a poster by the name of eclipse gave a full explanation, so I'm posting that as a guide here so other folks can play the song. (If you want, drop by the thread and give them some awards as thanks for the explanation!)

First off, you have to reset your story progress in the options as shown below.



Don't worry - this won't erase your high scores. Once you do this, you'll start playing the tutorial, you can skip it if you want. Once that's done, you'll be in the introduction area at the start of the game. Beat will say a few things, and when you gain control of her, walk to the left, and stand right next to the door, but do not go inside.



After a few minutes, Clef will yell at you to come in. Do that, then talk to your bandmates as normal. Play through the song, and when you're done, you'll be dropped off at the song selection menu.



Motherbound will now be available for playing, and it'll be the last selectable song in the list (press left at Empty Diary, or right after Synthion - True (From NOISZ SL).

This song only has one difficulty - Boing! - and it's not going to be easy. It's still less difficult than the hardest versions of the songs in the game, but it's tough enough to make you sweat. Good luck!