Unless you want physical CDs (or the stickers, posters and DLC items for D:DC and D:OS that are only in the boxed version of the anthology), the soundtracks are included in GOG's bonus content. As Kein posted above, they have a pay what you want deal for DD, where if you beat the current average you get BD, and if you are in the top 10% (currently at $13) you get D2:DC (and the associated extras).