A common cause of files not being listed by the selection screen on Yamaha arrangers, even if the screen is the correct one for that file type, is the length of the filename.
The maximum length that is displayed is 50 characters, and folders can be 50 characters long, but in the case of files this includes the final four characters for the file extension e.g. '.sty' or in your case '.mp3' ! If the total length is more than 50 characters then the keyboard won't display just the first 50 characters, it won't display the file or folder at all!
So you should check the files on a computer to determine the length of their filenames, and shorten them if necessary.
Files that are the wrong specification usually only generate an error message if you select one to load. They usually are still listed for selection if the name is not too long.
Regards
Ian