So one of the reasons I purchase games on GoG aside from lack of DRM, is I tend to find the higher price is offset by more extras bundled with a game.
I'm fairly certain 90+% of my curent library has a full manual, artwork, soundtrack, desktop wallpaper or a mixture.

For that reason the lack of Soundtrack for Undertale is conspicuous by its absence, especially considering Steam has it.

I wonder if it's some exclusivity deal. The Sountrack seems to be highly praised so having it exlusive to your store is a good selling point.

The soundtrack is in the game folder. Each song has mus_ in front of the title.