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The Rivevos Ephraim chelek 5:584:3 writes that by krias megila if one misses one word they are not yotzei(Mishna Brurah 690:5) accordingly it seems that certainly by parshas zachor which is a Torah obligation if one misses one word he is not yotzei,and it is like the Sefer Mikraei Kodesh siman 7 writes that it isn't any lower than megillah.Therefore one should be careful to listen to every single word.