Jukebox

The official soundtrack of Monolith was/is being created by Garoslaw, known also as ArcOfDream. The OST is created entirely in the software pxtone. You are able to find the releases on Bandcamp, split into the base-game OST and the RotP OST, which the latter will have music added to it when the update for RotP is released. You can find both releases here:

Base-game OST

RotP OST

On both releases there are bonus tracks, ranging from unused versions of songs, new versions created for the release, songs from other remix releases, a song created for Monorail, and a teaser track for yet unreleased tracks that will be added to the OST once added to the game.

Hub Jukebox
There is an in-game jukebox in the hub where you are able to change the music track that plays in the room until you start a run, which has tracks that you can play unlock as you encounter them. This jukebox can be purchased from Kleines. A Skully will sometimes hang out by the jukebox and will comment on the track playing if you've picked one, once you talk to them. A list of all tracks currently on the jukebox can be found below.