Talking during Kerias Hatorah:
Once the reader has begun to read the Torah it is forbidden for the congregants to speak. [Furthermore, some Poskim rule that even prior to the start of the reading, once the Sefer Torah is opened, it is forbidden to speak.]
The severity of the sin of speaking during Kerias Hatorah:
Speaking during Kerias Hatorah is a very severe sin. It causes one’s prayer to be an abomination and not be accepted by Hashem. It belittles the Torah and Hashem, and one’s punishment will be sure to come. It banishes the Shechina from the Shul, and causes death and poverty amongst the Jewish people. On the other hand one who respects the Torah and its reading causes that the Torah arouses Divine mercy upon him and his family in both spiritual and material matters.
Free Translation of the words of the Rebbe Rayatz on the severity of speaking during the Torah reading:
It is thus incumbent upon the reader of the Torah to be well versed in all the laws of Kerias Hatorah, as this matter touches one’s very soul. On this the verse states that life and death is in the hands of the tongue. Just as the reward is very great for one who reads the Torah properly, according to law, so too is the great and immense retribution against one who belittles the reading of the Torah properly and does not place vast effort to know all the tunes of the reading. One who does not do so is Heaven forbid considered like one who causes the masses, which are the listeners of his incorrect reading, to sin. On this the verse states that life and death is in the hands of the tongue. Now, this likewise applies to those that listen to the reading of the Torah, that they need to listen to the reading from a Sefer that is in their hands, and they need to hear each and every word. It is known the great and immense consequences in this world and the next to those who are not careful in this. It is explained in Sefarim that listening to the Torah reading properly affects the blessings one receives in Banaiy/children; Chayaiy/Health; and Mezoni/Parnasa. One who demeans the listening of the Torah is a brazen person which expels the king from his own palace. One who speaks to a friend is causing the masses to sin and increases Heaven forbid and Heaven forefend, death and poverty amongst the Jewish people.
Michaber 146/2; Sotah 39a; Zohar Vayakhel p. 206
 This prohibition applies according to all opinions regarding mundane words, as it is only regarding words of Torah in which there is a dispute.
 M”A 146/3 based on Gemara ibid; Gr”a; Soles Belula 146/3; M”B 146/4
Other opinions: Some Poskim rule the prohibition of speech only begins from the time that the reading commences. [Michaber ibid as explained in Beis Yosef; Rashi on Gemara ibid; Elya Raba 146/3; Mateh Yehuda146/3; Derech Hachaim 3; Shaareiy Efraim 4/11; Kaf Hachaim 146/8]
Biur Halacha 146/2 “Vehanachon”
Shelah Hakadosh p. 132
Rebbe Rayatz in translated Mamar brought next
Rebbe Rashab in Modaah Raba [printed in Siddur Im Dach p. 644]
Sefer Hamamarim 1937 p. 263