People tend to complain a lot about GOG not including games' soundtracks in the extras, but I think they fail to see GOG's perspective... from where they stand, I am positive that GOG would absolutely love to include every soundtrack they could possibly add as free extras to the games, and I think they tried all they could every time they were arranging the deals with the publishers of said games to have as many extras as possible, but it's something that's out of their hands. Sometimes OSTs are very expensive and involve copyright issues that not even the publisher can sort out, and in those cases GOG isn't allowed to include the soundtrack, as good as it may be.
It would be in GOG's best interest to add as many extras as possible to the games they sell, because that would make them some sort of 'definitive edition' compared to the versions sold on Steam, for example, so I would think they try all they can to get all the extras, with the publishers. If some extra or other aren't available it's most definitely not GOG's fault, it's the publisher that didn't give GOG permission to add those bonus contents for free with every game purchase. I know this sucks, and they kind of shot themselves in the foot when they started advertising this store as some place that offers every imaginable extra for all their games (they never really stated this, but I know a large group of people flocked over to GOG already expecting that, because they *do* include a whopping amount of free extra content with a big chunk of their -- older -- games), but, sometimes, it's just not possible for GOG to strike a perfect deal with the publishers, in which they can have all the extras they wanted in the first place.
And, well, there's always the copyright issue, sometimes the copyright holder of the OST for some game isn't the game's publisher (though I doubt this is the case with Bastion, since WB also has a music label and could easily release the games' soundtrack themselves, thus securing the copyright), some people at GOG might have bought the soundtrack for personal use, but they still couldn't upload it as an extra for any given game, because that would breach the legal contract with the publisher and would very likely end in them suing GOG. If, by any chance, the game's publisher also holds the rights to the soundtrack, then they will still not let go of that, because studios and corporations tend to think of money more than anything else, and giving the OST for free would mean less money when people can buy it somewhere else. This is especially true of games such as Bastion, that have an awesome soundtrack which has its own merits and stands by itself, independent of the game associated with it.
So, TL; DR, my point is that, although asking for something we want is a good thing and should be encouraged, we should try and give GOG some slack, because I'm positive they tried all they could whenever striking a release deal with any game's publisher, when it came to getting the maximum amount of bonus content for their games, and if that/those extra/s isn't/aren't available, most likely it's out of GOG's power to include them.