did you try renaming the FKD MP3 files then place them in the custom MP3 folder? could be name specific?
No the only thing I tried is what I posted a few posts up. They don't work with their default names in the Echoes custom music folder and they don't work renamed to particleillusionloop.mp3 in the Echoes music folder.
Didn't try naming them something like blahblahblah.mp3 and putting them in the custom folder, I might just for the heck of it though lol.