הלכה על ברכות 30:8

In addition to kavana, one must pronounce the words of Shema precisely, taking care not to slur any letter, harden soft sounds, or soften hard ones. Therefore, le-khatḥila, one should distinguish between an alef and an ayin, a khaf and a ḥet, a kamatz and a pataḥ, and a tzeirei and a segol (SA  61:14-23). The Sages say, “If one recites Shema and is careful to pronounce its letters exactly, Hell is cooled for him” (Berakhot 15b). Be-di’avad, if she did not recite Shema meticulously, she nevertheless fulfilled her obligation as long as she did not miss a word or a full syllable (SA 62:1; MB 1).