Sounds like they may be misnamed. That would cause the device to not read them correctly and possibly not play them.
Your best bet is to put them on a computer and check for the true file type. They may not actually be MP3 files (which should work on the phone). They could just have been named that. Not a shock. Happens a lot.
You can get yourself a program that can convert them. Your best bet would be to convert to another type (since they are already named with that extension) and then convert the new ones back to the other file type. So say convert the files to say a WAV file type (warning they are big) and then convert the WAV files back to MP3.
Me, I like the program Format Factory, but there are lots of them.
But before doing that I would test them on the computer to see if they work. There could be problems with the files other than being misnamed.