Whoops, looks like it is for sale. Since they were already in my library the prompt for the DLC was not showing up.
As for devs thinking GOG rejects paid soundtracks. Are there not many games on GOG that have paid soundtracks available?
I can only assume it has something to do with the fact that GOG used to add a bunch of goodies/extras for games, including soundtracks, which wouldn't be sold separately then.
Not sure how to search for OSTs (or any DLC) specifically so I can't really answer the question. I know some soundtracks are available, I know we have a bunch of games on the list whose soundtracks are missing - and more to come - and I know that there's often a lot of rights stuff involved when publishing soundtracks, which in addition to GOG being a smaller platform makes it seem "not worth it" to some.