It isn't as easy to do as it should be, but I've been able to get it to work. This is what I've done:
1. The game does not support mp3. You need a .ogg file. Ogg is like mp3, actually it has slightly better compression, but isn't as supported by players, but is basically free to use, while you need to pay to use mp3, so many games use .ogg over mp3.
Anyway, Audacity is a free, powerful music editor that can do the conversion. Download it here: http://audacity.sourceforge.net/
Install it. Run it. Open you music file. Then choose File -> save as OGG. Close Audacity.
2. You now have the ogg file that the game wants, however, the song title / artist and comment don't get copied over. Open the ogg in your favorite player and see it it will let you edit the media info / ID3 tag in the ogg file. If not, Winamp should work or VLC ( http://www.videolan.org/vlc/
) should also work.
The game will display the song title and artist and the comment. Either copy from the original music file, or add your own info ( some people like to put their track name in comment field to 'prevent' other people from trying to re-use their music). Close the music player after saving the changes.
3. In the advanced editing options, choose custom music, and then choose the ogg file.
The game will convert the .ogg into a .mux file, and put it in the ManiaPlanet\Media\Musics directory in your My Documents directory.
Music will now work for you, but not for other people as they don't have the .mux file
To get the music working for other people / servers:
4. Copy the .mux file from there and post it online somewhere it can be directly downloaded. You can use webspace for this as long as there is no initial downloading page. Dropbox seems to work if using the public dropbox directory.
5. Remove the .mux file from the music directory. This is so we can test if downloading works. If you hear music in at the end of these steps, it will be because the download worked. If not, something wasn't done correctly.
6. Create a new text file in the music directory that had the .mux file. Paste into the text file the URL of the .mux file that you uploaded. It should be the first and only line in that text file. Save and close the text file.
7. Rename the text file to be the same name of the mux file, however, the extension should now be ".mux.loc" . So if you music file was "music.mux" the text file should now be "music.mux.loc". 'loc' is short for location.
8. Go back into the track editor, and re-choose the custom music. This will now read the text file with the URL and put the URL into your track file. Save your track, and then validate / play it.
10. You should now hear your custom music even though you don't have the .mux file in the music directory anymore. It may not play the first time, but after 1 or 2 restarts it should play (it needs some time to download the music).
If it doesn't play, either the URL doesn't directly download the .mux file (for example, it shows a webpage with a download link, which the game won't understand), or the .mux.loc file is somehow incorrect.
Once this works, giving your track file to other people or uploading it to mania-exchange will have their game automatically download the music when they play the track.