What games on GOG let you play your own music within the game?

Civilization 4
The Surge (though this is only in certain areas).

Only two I can think of off the top of my head.

Civilization 4; with the notable asterisk that it requires specific formats and tends to be blind to anything outside it's home directory.

Rebel Galaxy has an option in the launcher for custom music. There's actually several songs from Crossout that mix very well with the game too. (If you want the songs i compiled let me know i'll see if i can't share them)

Symphony: Not only does it allow you to listen to your own music, it actually creates custom levels out of it.

In the same vein, there is also Beat Hazard and its sequel, though I haven't played them.

(As a sidenote, I remember it was not too hard to replace the music in Painkiller Black, but it wasn't a feature of the game, you actually had to mess with the music files, so probably not what you're looking for.)

Any others?