It is not necessary to be particular in the pronunciation of the words. However some rule this only applies if the mistaken pronunciation does not change the meaning of the word, such as one who said Yehudiyim versus Yehudim. If however the mistake changes the meaning of the word, he must repeat that word in the correct pronunciation. [Practically we rule like this latter opinion. If he already completed the reading it is to be repeated without a blessing.]
If no one knows how to read the Megillah with the correct punctuation: If there is no one available that knows the correct punctuation for the Megillah reading, the Megillah is to be read without a blessing. Alternatively, in such a case one can appoint another person to read quietly from a Chumash to the reader of the Megillah, and the reader repeats the words with the correct punctuation. [In such a case a blessing is recited on the reading.] Nevertheless in such a case, after the conclusion of the reading, the reader of the Chumash is to repeat the reading for himself from a Kosher Megillah. [Alternatively someone is to read along in a Chumash and correct the reader in all cases of mispronunciation and in such a case a blessing may be recited.]
 See Piskeiy Teshuvos 690/3 for various customs on this matter; For a list of words that have a questionable pronunciation-see Halacha 23F!
 Otherwise it is considered as if that word was not read, and one thus did not fulfill the Mitzvah. For example if he read Menufal instead of Nafal or Meyushav instead of Yashav. [M”A 690/16; Elya Raba 690/9; M”B 690/51]
 Peri Chadash 690; Kaf Hachaim 690/86
 Biur Halacha 690 “Umidakdikin”
 Shaareiy Teshuvah 690/1; Kaf Hachaim 690/30-31
 Shaar Efraim 6/58 brought in Shaareiy Teshuvah ibid
 Shaar Efraim ibid
The reason: As the reader of the Chumash is most probably too focused on saying over the words to the reader of the Megillah and he thus is unable to listen to the repeated reading in the Megillah. [ibid]
 Seemingly in such a case the reader of the Chumash is not required to repeat the reading, as he is paying attention to every word of the reader.