What region is the soundtrack selling separately in? It's included in all retail versions in Canada. Collectors, standard, gog, and steam (should be in the Witcher 3 folder in your steam apps directory).
I bought it from amazon Germany and it was about 45 EURO compared to 40 EURO as stand alone game.
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It was also made available for Steam customers. Check your Steam folder under steamapps/common/The Witcher 3/soundtrack
The only thing Gog got extra on this was 3 bonus tracks that were made available at the time of ordering.
When you buy any PC version other than Steam, you were given a Gog code. That gave you access to the OST. I don't know why you paid extra, but it shouldn't have been for the soundtrack.
The bundle itself had a CD attached. The GOG code was not mentioned separately in the advertisement, only -with official CD soundtrack -. In fact, GOG was never mentioned at all. It was just the advertisement for hard copy + soundtrack and frankly, not even installing GOG was mentioned anywhere (not even review articles) but instead 'no online access needed', which , in retropsect, was false advertisement since GOG pops up right away after starting the install. But this is a different matter and off topic.
I lookd at many OSTs from games and everytime there was some track or the other missing. As a customer, who is that old to expect complete products, I feel that is cheating on my money. I just thought, it is nice and helpful to tell people, that this soundtrack might not be complete. As far as I can remember, the battle music when fighting contract bosses is not included.
Don't get me wrong, I am not a disgruntled customer, I am an enthusiast for soundtracks and maybe someone feels like me and cares if the OST is complete or not.