You don't have to fight him to get your mp3 player back, fighting will just escalate the problem. We aren't cavemen anymore, we can settle things peacefully. Just go up to him and be like "hey, have you seen my mp3 player? I left it on the fridge." if he says no (which he probably will) just say "damn, well I was going to report it to the police as stolen, are you sure you haven't seen it? You could get suspended for stealing yknow?" It could be utter BS, just tell him subtly that you know he took it, and he will most likely give it up.