No background music can be heard in the game.
Other sounds work properly.
Is this a bug or has the soundtrack been removed from the game?

Maybe the music is turned off in the sound settings? I had music playing that I turned off.

If you're on Windows 8.1 run game in Windows 7 mode (instead of Windows 8.1/10 mode), than music works.

I am currently having the same issue running the game on Linux: Actually the issue is very well known in Steam, and the fix of running it on Windows7 mode works on Steam, but not the GOG version it seems: All modes work for sound, but there is no music.