Well, with the update of the Steam interface a few months ago, I just realized that apparently Steam does not recognize the DLCs as a soundtrack.
I think this is because on the Steam page they are listed as DLC and not as a soundtrack, since these should have the purple label of a piece of music in the Steam store.
So it is not possible to play it within steam.
I really hope this problem is solved soon or at least have a prompt response from the developer.