Why doesn't GOG sell a version of the game with the soundtrack? Steam and Humble both have a version that includes the soundtrack.

I know this isn't the same thing, but the soundtrack is an unlockable... in game. Also (off topic), as another gog-er pointed out, where is the manual?

hmm, so, when you unlock the soundtrack, you can then download it and
copy onto your mp3 player to hear the music while outside?

Or is it something else?
Really don't know how this would work.

Unlocks like this usually only work in game with a built in player. I'd imagine this is the way Pier Solar works too. Only the Humble and Steam versions have the option of buying the soundtrack in mp3.