To put my own music into the game through SmashMusic, I've been using the method where you take the name of a default song, found in SmashMusic, and then rename the custom song with that default song's name.
For some reason, a couple of the songs that I've been adding aren't showing up in-game.
I think it may be an issue where some of the default songs simply cannot be replaced, but idk. Any help appreciated.
I use smashcustommusic.com to download songs that I want in my mods but there are a few songs that once I put in game don't play, what I suggest you to do is try to add new songs slots for the stages and menu its easier than you think and also it has an option where you can listen to the song before build the mod. I used this tutorial I hope it helps you https://youtu.be/GnxcLg3RNVQ
and as I say there are songs that just don't play in game