It is very disrespectful how they are acting.
Everyone has their own unique way of doing things! They should know that by now! They are old enough. I suggest you just ignore them because they are being nosy. If you do that, then they will know you're not bothered and will stop. If that doesn't work however, you should try this useful method below:
1) first, just be normal and wait for a time when they start mocking you about that again.
2)walk up to them, and look serious about it - you are! You want this to stop and it will.
3) Keep walking forwards, and take them by the shoulders gently, but seriously. Walk forwards and make them walk backwards until you corner them. Do this whether there are other people or not - don't worry about them.
4) Tell them the truth - tell them how you feel and how strongly and gravely you feel about this matter. Make eye contact and hold their shoulders while you talk.
5) Say to them, they are being very disrespectful of your feelings and your property, because they took your MP3 player - a source of equipment that DOES NOT belong to them.
6) finally, say to them that you are very hurt and angry by their comments,and nosiness. Use a softer tone, and tell them to say sorry to you or maybe make them confess that they were doing the wrong thing, and make sure they say that it will never happen again.
If you do this, I'm sure they will feel that they have done wrong. Even if they didn't it'll probably never happen again because you have shown that you can stand up for yourself and not listen to their babyish behaviour all day long - keep your chin high, hopes up!