Welcome to Justanswer.com, my name is ***** ***** I will do my best to assist you.
Itunes may need a repair. Or, it may not like the way the mp3 files are encoded. As Mp3 is a very general term for a type of file that can be encoded in many different ways.
It may be best if I connect to your mac from here, so I can try and fix the issue directly on screen. And once I have figured out what the issue was.. I can then make sure you know exactly how to fix this should it happen again.
Im sending that offer to you now, once accepted or rejected we can continue.