Seems like you are trying to play very high quality songs or their format is incorrect.
Format the mp3 player's internal memory in FAT file system. (Format is same as erasing the entire memory contents). Before copying your songs encode them in the mp3 format with bitrate 128kbps max. Try 64 kbps if it still does not work. Remember to copy your songs to the memory directly or to its predefined folders. Try not to keep folders to just one.
You can try these free software samples to encode your songs (if you already do not have one):FreeMP3 RIP
Please note that there are numerous other software for encoding tasks. I am providing the above links for reference only and have chosen them randomly. I am in no way affiliated with their developers and take no responsibility for their actions.