Watched a playthrough of the unrated version of the game and liked it enough to go looking for both the game and soundtrack. Alas, Agony is not available to me at all. I'm Mac only at this point, so I gotta wait for a conversion that may never happen... and, while I'd like to enjoy the unsettling music during the wait, I can't buy the soundtrack here without owning the game... Anyone know why that is?

Unfortunately, that's normal for any bonus content like soundtracks, wallpapers etc. You have to own the games to buy them. And since it's like this with all of the games that I know of, it seems to be a decision of GOG, not of the particular developers. I don't know why, though. I don't like it either. They forfeit the extra money from people that only want the soundtracks etc.