The assignment requires you to take a position and argue this position. It would be best if you do it by yourself, because I don't know what your position is.
But I can give you some tips. Supposed you agree with this statement. You could then start with an introduction of how listening to MP3s affect people: it gives a sign to the others that they should not talk to you (since you can't hear them), and makes you less attentive (usually when we walk around we also look around so that we're not bored, but listening to MP3 gives you something to do so you're not that bored. This also means that you are at a greater risk of having a car accident or walking into something.
These are all disadvantages of listening to MP3s and if one considers them important, one might decide not to listen to MP3s in public.
