The Shulchan Aruch writes (Siman 62) that there is a Mitzva to be very precise in the pronunciation of the words during the Shema recitation, but nevertheless one who did not recite the words with perfect precision has nevertheless fulfilled his obligation. The Mishna Berura (commentary by the "Chafetz Chayim" – Rabbi Yisrael Meir Kagan, Lithuania, 1835-1933) clarifies that the Shulchan Aruch does not refer here to the mispronunciation of words, such as omitting a letter or reciting the wrong vowel sound. Somebody who actually mispronounces a word clearly does not fulfill the obligation to recite Shema. Rather, the Shulchan Aruch speaks here of meticulous precision in enunciating each letter and sound with all its particular nuances. For example, one should preferably make a point to carefully enunciate the "z" sound in the word "Tizkeru," but nevertheless he fulfills his obligation even if he says it normally, without emphasizing the "z" sound.
Chacham Benzion Abba Shaul (Jerusalem, 1924-1998), in his discussion of this subject, goes through several letters of the Hebrew alphabet that one must ensure to pronounce carefully during the recitation of Shema. There is a difference, for example, between the letters "Tet" and "Tav." Although they are both pronounced with the "t" sound, the "Tet" is a stronger sound than "Tav." Similarly, one should carefully distinguish between the letters "Kaf" and "Kof." The letter "Kof" should be pronounced as a stronger sound, produced from the back of the throat. A difference must also be made between the "Samech" and a "Tzadi." The "Samech" is pronounced as simply an "s" sound, whereas the "Tzadi" represents a stronger "s" sound, to the point where many people today recite it as "tz," which is incorrect. There is a difference as well between the letter "Gimmel" with a Dagesh (dot in the middle of the letter) and without a Dagesh. When written with a Dagesh, the letter "Gimmel" is pronounced as a "g" sound (as in the word "good"), while in the absence of a Dagesh, it is pronounced like an Israeli "Reish."
One should preferably make a point of following these rules of pronunciation when reciting Shema.
There is some debate concerning the proper pronunciation of the letter "Bet" when it appears without the Dagesh. The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909), in his work Rav Pe'alim, mentions that the practice in Baghdad was to recite this letter with a "b" sound, just as one does when it has a Dagesh, and he vehemently denounces this practice, claiming that when written without a Dagesh the letter is pronounced "v." Chacham Benzion Abba Shaul proves from the Beit Yosef (work by Rabbi Yosef Karo, author of the Shulchan Aruch) that this is, indeed, the correct pronunciation – to pronounce the letter as a "v" sound if there is no Dagesh. The custom of the Jews of Halab (Allepo) is not to make a difference between a Bet with a dagesh and Bet without a dagesh.
The Talmud states that if one recites Shema very slowly and carefully to properly pronounce all the words, then Gehinam will be "cooled" for him. The Mishna Berura explains that one who controls the natural tendency to rush through the recitation of Shema is rewarded by having the intense heat of Gehinam come under control so that his soul will not have to endure harsh punishment there in Gehinam.
