Hey there, sichu4n.
Do you get any specific errors when you try to play any of those files? Are you able to copy them to your computer and see if they work afterwards? Make sure that you click on the icon in the bottom right corner (next to the clock) and eject the flash drive (Safely Remove Hardware and Eject Media) before you unplug it from the USB port. Other than that you could try reformatting it, to see if the same thing happens afterwards.
Unfortunately there's not much you can do in this situation. You could try the same thing with different devices as well, but if the same thing happens, you should contact the reseller you got it from or the manufacturer and ask for further assistance or if you can get it replaced.
Hope that...