http://braveburgerrecords.bandcamp.com/album/reus-soundtrack

I don't own the game, but I suspect that the music may be amongst the installed files!

http://www.fileinfo.com/extension/fsb

I guess it's similar to dynamic music used by Unreal, and certain sections play when certain events are happening.
You might be able to open up the file with Audacity and cut out snippets of the music/export to WAV or MP3.

I enjoyed when we'd get full soundtracks with GOG purchases instead of teasers. Granted, I still purchase much of this music but it was a really awesome thing. I suppose good things don't last forever. -Adam

It appears to have all the tracks listed by Joni van der Leeuw on Bandcamp and a few little extra parts.
I converted it using a Sound File Converter and cut it up by ear.
Some of the tracks seem to be shorter then their listings on Bandcamp, so I will give both lots a listen when I get time and see what matches up and what does not.

EDIT:: They are in a different order in the Music_bank02 file.

That falls on artists now wanting to sell their music seperately and at extra cost, not so much on GOG.

It's partially on GOG. Deals must be brokered. And even if the reason was that music was an endangered species, it still doesn't change that it was more enjoyable to get the full soundtracks. -Adam

Seems that the game uses FMOD which stores the audio assets in sound bank files (FSB stands for FMOD Sound Bank). There's apparently a tool that allows to extract these files, I haven't tried it myself, though (and frankly I'm not sure it's legal to actually extract FSB files but I know that that's not gonna keep anyone from doing it).