@Raphael-Bueno-Hatencia If music order is hardcoded in your theme system A = music A, system B = music B etc... you can't randomize the music.
If you use a global music folder (as done on recalbox theme) or put your musics in music folder of your share partition, randomize will be automatic on 4.1.0 recalbox release.
I've seen different threads where people have done that. Use the search function and you'll find it.
Nachtgarm didn't give any instructions, it was just an idea.