Patch 1.19 is live in Japan and (Apprently) North America now. I did a Google Translate of the SC patch news site:
Add custom sound feature
<MP3 import method> 1. Prepare a USB memory 2. Connect USB memory to PC and create dedicated folder for Fire Pro Create "MUSIC" folder, "FPW" folder in MUSIC folder 3. Save the MP3 you want to use in the FPW folder 4. Connect USB memory with saved MP3 to PS4 5. Start Fire Pro World and select "Import BGM" from the option menu
I've already downloaded and mass cropped 350+ songs so that 40 file cap is a bit of a pain but glad to have the patch and the specs for the files. I'm in work now but I'll have a go this evening (BST) and fill everyone in unless someone beats me to it.