Shouldn't the CIV4 version on GOG have a download with the original soundtrack of the game?

Not all games do. Even if you look at the store page, you won't see it in the goodies section.

Civ4 does have the soundtrack in the game directory as MP3s, so just play it directly.

Thank you, I had not thought of looking for the mp3 files inside the installation folder.

Always worth checking this. The idea of selling the soundtrack separately is a relatively recent one, and the vast majority of older games just have the soundtrack files sitting unencrypted in the installation folder. Heck, a lot of recent games are like this too.

it's not the full ost, but christopher tin's music is available on spotify and you can listen to baba yetu and sogno di volare (civ6's theme) there

A lot of the old computer games that came on CD/DVD could be put into CD players or boom boxes and the music played just like a regular music CD. I can remember doing that with games like Might and Magic 6.