Rav Haim Palachi (Turkey, 1788-1869), in his work Mo’ed Le’chol Hai, records a custom that the congregation recites the Shema prayer on Shabuot morning loudly and in unison. One reason for this custom is to ensure that everyone stays awake and alert for the recitation. After remaining awake studying Torah throughout the night, people are naturally drowsy on Shabuot morning, and they thus run the risk of sleeping during the Shema recitation, thereby neglecting the Torah obligation to read the Shema. It is therefore proper for the congregation to make a point of reading the text of Shema out loud together to help keep all the congregants awake for the Shema recitation.
There is, however, an additional reason for this custom, as Rav Haim Palachi notes. The Ten Commandments, which we read in the synagogue on Shabuot morning, are all alluded to in the text of Shema. The first commandment – "I am Hashem your God" – clearly corresponds to the first verse of Shema: "Hear, O Israel, Hashem our God – Hashem is one." The prohibition against idolatry, which comprises the second commandment, is mentioned explicitly in the second paragraph of Shema ("Pen Yifte Lebabchem Ve’sartem Va’abadetem Elohim Aherim"). The command to "love Hashem your God with all your heart," which we read in Shema, parallels the prohibition against uttering God’s Name in vain, as one who loves the Almighty does not insult Him by freely uttering His Name. The words "Le’ma’an Tizkeru" ("in order that you remember") which we recite in Shema bring to mind the fourth commandment, the Misva of Shabbat – "Zachor Et Yom Ha’Shabbat Le’kadesho" ("Remember the day of Shabbat, to make it sacred"). We read in Shema of the promise of long life ("Lema’an Yirbu Yemechem"), which is the reward promised for observing the commandment of honoring parents ("Kabed Et Abicha…Le’ma’an Ya’arichun Yamecha"). The prohibition against murder is alluded to in the warning "Va’abadetem Mehera" ("you will be quickly destroyed"), which refers to death in retribution for murder (as the Talmud Yerushalmi states, "Mann De’ketil Yitkatal" – "One who kills shall be killed"). The verse toward the end of Shema which warns against straying after one’s eyes ("Ve’lo Taturu") alludes to the sin of adultery. Theft is indicated by the description in Shema of Beneh Yisrael’s harvest – "Ve’asafta Deganecha" ("You shall gather your grain") – implicitly warning against taking other people’s produce. In the final verse of Shema, we testify to God’s having taken us from Egypt, and it thus corresponds to the prohibition against bearing false witness. Finally, the command to affix a Mezuza to the doorpost of one’s home, which appears twice in Shema, alludes to the tenth commandment, "Lo Tahmod" ("You shall not covet"), which forbids coveting one’s fellow’s home.
Thus, for this reason, too, it is appropriate for the congregation to recite the Shema out loud on Shabuot morning, as it bears unique significance on Shabuot, when we celebrate the event of Matan Torah and read the Ten Commandments. And while it is certainly proper to have these allusions in mind whenever one reads Shema, it is especially worthwhile to concentrate on them when reading the Shema on Shabuot.
Summary: It is proper for the congregation to recite the full text of the Shema aloud and in unison during Shaharit on Shabuot morning.