If someone didn't hear words (or perhaps even an entire verse) from Parshas Zachor or Parshas Parah, are they yotzee b'dieved? I'm specifically asking in this case since Parshas Zachor is d'oraisa and there are some who say Parshas Parah is also d'oraisa. However I suppose the question could apply as well to Parshas Shekalim or Parshas HaChodesh.
The Rivevos Ephraim chelek 5:584:3 writes that, since if one misses one word of krias megila 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: like the Sefer Mikraei Kodesh siman 7 writes, it isn't any lower than megillah. Therefore one should be careful to listen to every single word.