Those are already included in the extras section so people who already purchased this might not be interested.
The dev however have posted a download link on his discord and allowed me to post this here for everybody. Maybe someobody who does not own the game yet gets interested or wants to support such nice behaviour :)

So here is the download link to the soundtrack and wallpapers of the game:
https://www.dropbox.com/sh/dlkv5wh3k3gcvvl/AABFhx9D2lBgho6cYrGx0k6Pa?dl=0