Hi Phil, the first I suggest you to do is stopping using your MP3 play first. This action can avoid data overwritten. Now I will explain the details to you.
Sometimes formatting is just like deleting files, but actually no matter what action you've taken, the files are not really gone. A little complicated, right? Let me tell you in a simple way.
Like this, you put a file named "A" into your MP3 player, then you format your MP3 player or delete "A" from your MP3 player, the operating system will not really delete file "A"-it just frees up the space of file "A" so that the space can be used by any other new files you will put into the MP3 player in the future. Now if you put a new file named "B" into your MP3 player, "B" may probably take the space which was belonged to "A", this is data overwritten situation. If this happens, you may lose the chance of recovering your files. That's why I ask you to stop using your MP3 player.
Now you can start to search methods or apps on google to recover your files. But no matter what kind of methods or apps you are using, make sure they won't do harm to your "A" file (I mean they won't cause the data overwritten situation).
If you are using apps to recover your MP3 player files, you need to know that the apps can only be used on your computer. Here offer you two posts, you can see if it can help you or not. (I don't know what kind of computer OS you are using, Mac OS or Windows PC, so I offer you two post. Good luck.)
How to Recover Lost Data from USB on Mac
(This can be used on Mac OS and be used for storage device like MP3 player.)
uFlysoft Photo Recovery Recover lost Photos Videos Audios easily and...
(This can be used on Windows PC and be also used for storage device like MP3 player.)