Music files have to be in the .ogg vorbis format. I included the "vorbisgain.exe" program, which helps removing distorsion caused by too high volumes (it's a known SFML bug).

Simply place your music file in this folder, then run vorbisgain.exe using the file as a parameter (either via command line or batch file), like this:

vorbisgain.exe drFinkelfracken.ogg

After that, you're ready to make a .json file for the music (it is required).

Copy paste an existing .json file, it's easier.

The "id" member requires you to invent an unique id for the music data. It doesn't have to be the same as the music file. It mustn't be the same as any other id.

The "file_name" member is the name of the music file with the .ogg extension.

Add your music information to make it show up in the menu.

The "segments" array contains objects with "time" members - every value is in seconds, and represents the start of a segment in the song which is suitable to gameplay.

If your song has a long intro, or different parts that you want to hear in random sequences, you can add the corresponding second as a new object in the "segments" array, like I've done for the existing songs.

If you want the full songs to always play, add a single object, with "time" equal to 0.

You can easily convert any music/sound file by using the program "foobar2000", which can be easily found on Google.