The MP3 files requirements as follows:
Sample Rate: 48Khz, 44.1khz, 32khz, 22.05khz, 16khz, 11.025khz, 8khz.
Bit Rate: 128kpbs, 64kbps, 32kbps.
Mono type.
You also can find the answer from Playlist builder online help document.

Released for:Schneider Electric UK