Hello rattler,
I think I can offer some guidance ---
Playback of mp3 files is described on p. 28 and p. 67 of the Operator's manual.
Notice that the mp3 can be either 44.1khz or 48khz sampling, so that shouldn't be a problem.
Basically you need to navigate to the USB stick, and then the folder that you saved your mp3 into.
Insert your USB stick, and then from the Main screen, just tap on the "Audio" section, right next to MIDI.
You'll see your USB1 stick on the left hand side of the file selection, tap on USB1. Now you'll see your folders.
Tap on the folder you saved your mp3 into --- or if they're in the root of the drive, you'll see them listed.
I created a specific folder called "Audio", where I store my mp3 files, and then just tap on the song, hit the exit button, and back on the Main screen
You can play the song by tapping the Audio play button, just below the MIDI button. Or, you can tap the PLAYER button, and you'll have controls for both formats.
By using the PLAYER button, you'll also see that you can use the Voice Cancel, Time stretch, Pitch shift and Repeat functions!
Hope that helps, Regards, Jerry