grimwerk: I've figured out how to replace the music, and it's pretty straightforward.

Sadly, whenever I try to post that solution here, the website complains:
You cannot post any links at this moment.

Even though my solution contains no links.

I'll edit this post with the solution once GOG resolves the posting problem.

grimwerk: You can find the FlatOut music folder here:
...\FlatOut\data\music

It contains all the music files for the game, in ogg format. It also contains a file called playlist_0.bed . Open playlist_0.bed in a text editor, and you will see that it is a kind of index to all the music played in game.

So:
1. Convert your new music files to ogg format (if they are not already). I used VLC to do this.
2. Copy those files to the music directory.
3. Edit playlist_0.bed to refer to the new music files. (Make a backup copy of playlist_0.bed, first!)

The Title = line changes the music played during the game title screen.
The 26 entries in the Loop = section are played at random as you drive. You can change the music files, and also the artist and song info that appears in game. Be careful to preserve the existing formatting of the playlist_0.bed. If you change the number of songs in the list, be sure to change the Count = 26 line to match!

If any of this is unclear, post here or feel free to PM me!

grimwerk: You can find the FlatOut music folder here:
...\FlatOut\data\music