The halachah is, if the one reading Megillah suddenly loses his voice in the middle of reading Megillah for the tzibbur, the replacement ba’al koreh doesn’t need to start over from the beginning, rather, he can continue from where the original ba’al koreh stopped (certainly, he doesn’t need to repeat the berachos) as doing so is tircha detzibbur [burdensome on the congregation] (Mishnah Berurah 692:2). However, the Sha’arei Teshuvah (s.k. 2) points out, this is only in a year when everyone is fasting, when Purim falls on motzei Shabbos, however, if the majority of the tzibbur agree that he should start over, then it’s more ideal to start again:שלא יהיה נפשם עגומה בקריאה לחצאין – “in order that they won’t be upset with reading the Megillah in halves”. Presumably, the same thing would be in our case of the hatzolah member as well.
