Check if you can play those on another player or a computer.
Check if those files are realy MP3 or MP4 files. Just changing the extention DOES NOT WORK. The extention must reflect the actual encoding of the file. It's possible that your player only use the extention and not the information from the file header to decide what decoding to perform.
If the actual format is not correct: On a computer, use some convertion application to convert your music to a format that is supported by your player.
Electro, December 2013
