Lets get back to basics.
What is the format of the files you are trying to play?
IE
WAV, MP3 please include sampling and bit rates.
What is the source of the digital files?
IE
iTunes, Amazon, CD rip etc
Is protection applied to the files?
IE
DRM
If you look through the manual you will probably find that your audio files have to be of a specific format like WAV 44.1kHz 16 bit and if this is not using the correct file type and format the system will not play them.
If you are unsure, rip a CD but ensure it rips to WAV, check the format of the ripped file using MP3Tag and correct is needed, then load one file on your memory stick after removing all other files and see how it goes.
Finally never insert a flash drive or remove one whilst the keyboard isturned on, on a computer always use the Eject function, never, ever simply pull out a flash drive.