Related על ברכות 2:3

One who reads the Megillah out of order does not fill [his obligation], and so too with Hallel and so too with tefillah (i.e., the Amidah), and so too with the recitation of the Shema. [If] the [public] reader of the Megillah errors or skips over one of its verses, he does not go back and read the verse by itself, but he begins from the same verse and continues until the very end [of the Megillah], and so too with Hallel, and so too with tefillah, and so too with the recitation of the Shema. One who enters the synagogue and found that they had [already] read half [of the Megillah], and he finishes with them, he does not go back and read it from the beginning until the place [where they were when he arrived], but rather he starts from the beginning and continues until the very end, and so too with Hallel, and so too with tefillah, and so too with the recitation of the Shema.

One that reads the Shema backwards [i.e. out of order] does not fulfill his obligation [of saying the Shema]. And the same [applies] to Hallel, to prayer [of Shemoneh Esreh], and to the Megillah [of Esther].

One who was reading the Shema and made a mistake, and skipped in it one verse, [he] should not begin to read that verse by itself, but rather should go back to that verse and complete [the Shema, from that point on] until the end. And the same [applies] to Hallel, to prayer [of Shemoneh Esreh], and to the Megillah [of Esther]. One who entered a synagogue and found that [the congregation] has read half of it (i.e. the Shema) and [he] completed [the remaining half of the Shema] with them, [he] should not go back and read it (i.e. the Shema) from the beginning until that place, but rather [he] should begin from the beginning and complete it until the end. And the same [applies] to Hallel, to Tefillah (prayer) [of Shemoneh Esreh], and to the Megillah [of Esther].

One who reads the Shema and makes a mistake, and [he] does not know where he made the mistake, [he] should go back to the beginning of the [first] paragraph. If he made a mistake in the middle of a paragraph, he should go back to the beginning of [that] paragraph. If he made a mistake between the first [verse that mentions] writing and the second [verse that mentions] writing, he should go back to the first [verse that mentions] writing.

“A man who had a seminal emission (Baal Keri) who does not have water to dip in may read the Shema, but he may not [read it loud enough so that he can] hear [himself talking] with his own ear, and does not say the Beracha (blessing) not before it and not after it.” [These are] the words of Rebbi Meir. And the Chachamim (Sages) say, ”He may read the Shema and he may [read it loud enough so that he can] hear [himself talking] with his own ear, and he says the Beracha [both] before it and after it.” Rebbi Meir said, “One time we were sitting in the Bet Midrash (Study Hall) in front of Rebbi Akiva and we were reading the Shema, but we were not saying it loud enough to be able to hear ourselves, because of one inquisitor who was standing by the door.” They (i.e. Chachamim) said [back] to him, “The time of danger is not a proof.”