I guess if you ain't the receiver then you gotta face others playlist :P Or Nintendo just use the default one when battling online. Although can't confirm yet cuz I'm busy setting everything in My Music ... which is freaking long to do with the numbers of stage ...
Yeah but it is frustrating because it didn't work that way in Smash 4. Back then each player had their own music settings, they could even be listening to different songs in the same match. It took me several hours to finish setting up My Music, I want it to work!
Also, I know everyone's listened to this a couple million times already, but my god it's so amazing