1. One must read/hear the entire Megillah, from beginning to end. If the person omitted even one word that he did not read/hear — the majority of poskim rule that the person did not fulfill his obligation (Shulchan Aruch and Mishnah Berurah 690).
2. Therefore each individual who is listening should have their own Megilla, even if it is not on parchment, so that if he misses a word or two he will be able to fill-in the words he missed from the Megillah in his hands. For instance, while the children make noise at hearing the word “Haman,” the person can read the words he did not hear [as long as it is only a few words] and continue to hear the rest from the Reader. [However, the person will have to read quickly so that he will manage to say both the words he missed and the words the Reader said in the meantime, while the person was catching up. The general rule in all this is that there cannot be even a single word that the person did not hear from the Reader or that he did not say himself. Also, in the end, all the words of the Megillah must have been said in sequence] (ibid.).
