We have updated all our soundtrack DLCs to the new Music format for Soundtrack. You can now purchase soundtracks without purchasing the base game. Have fun!
For the users who already own the soundtrack as DLC, the update is automatic and you will find the soundtrack and the other music as a new section in the Steam client library, from which you can access the contents.
https://store.steampowered.com/app/1115480/Idol_Magical_Girl_Chiru_Chiru_Michiru_Original_Soundtrack/