It was a pain in the butt trying to figure out how to get custom music to work through the workshop; there's a lot of misinformation online about how to do it.

If you want to make your own track and add custom music, this is what you seem to have to do:

1. Save your ".ogg" music file in Documents\Maniaplanet\Media\Musics
2. Edit your track and select that file as your custom music.
3. Make sure a ".mux" file appears with the same name as your music file in Documents\Maniaplanet\Media\Musics, if it doesn't try relaunching the game and playing the track.
4. Exit the game. Remove or delete your ".ogg" file from Documents\Maniaplanet\Media\Musics
5. Open the game, edit the track, select custom music and click the name of your music file (it should show up without you needing to browse to it - I think the reason you need to do this is it forces the game to use your mux file), and save the track.
6. Upload a copy of your Mux file online, such as to DropBox or Google Drive, make it publicly accessible, and get a DIRECT download link for it (if you navigate to your link it should cause your browser to immediately download the file)
7. Create a text (.txt) file with the same name as your song in Documents\Maniaplanet\Media\Musics , and in the text file paste that Direct download link, then save and close the file.
8. Change the file extension from ".txt" to ".mux.loc"
9. Open Trackmania, edit your track, and save your track (not sure if this step is necessary again, but just do it for good luck.
10. Upload your track to the steam workshop using the in-game option to do so (the option should be on the screen before you select a track to edit).