The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1835-1909), in Parashat Emor, writes that after one recites a Beracha over food, he must not talk until he swallows at least part of the food. Preferably, he adds, one should refrain from speaking until he eats a complete Ke-zayit (volume of an olive). This is, however, but an added measure of stringency. According to the strict Halacha, one must refrain from speaking after reciting a Beracha over food only until he swallows some of the food.
If one spoke before swallowing some of the food, then so long as he had at least placed the food in his mouth before speaking, he need not repeat the Beracha. However, if a person recited a Beracha and spoke before he even put the food in his mouth, he must repeat the Beracha.
If a person recited a Beracha over food and then hears a Beracha or Kaddish, he may answer "Amen" if he had already put the food in his mouth. Even if he has yet to swallow any of the food, and it is thus forbidden for him to speak, an exception is made allowing him to answer "Amen," provided that the food had already entered his mouth. Before one puts the food in his mouth, however, one may not answer "Amen" to a Beracha or Kaddish after reciting a Beracha over food.
The Ben Ish Chai also discusses a case where one recites the Beracha of HaMotzih on behalf of everyone at the table (as is commonly the case at a Shabbat meal), and he incorrectly begins to speak after reciting the Beracha, before partaking of the bread. (In the interest of clarity, we will speak of a situation where a host recited the Beracha on behalf of guests.) The host, as discussed, must then recite a new Beracha before he eats. The question arises as to whether the guests, who are relying on his recitation of the Beracha, must also recite a new Beracha. The Ben Ish Chai rules that if the guests remain silent before partaking of the bread, then they need not recite a new Beracha. Even though the host, who recited the Beracha on their behalf, "lost" the Beracha by speaking before eating, the guests may still rely on his initial recitation and do not require a new Beracha.
A different question arises in the reverse situation, where the host immediately partook of the bread without speaking in the interim, but the guests began speaking before they partook of their pieces of bread. Halacha establishes that if they began speaking only after the host partook of his bread, they need not recite a new Beracha, even though they spoke before eating.
Summary:
1) One who recites a Beracha over food must not speak until he swallows at least part of the food.
2) However, if one hears a Beracha or Kaddish after placing the food in his mouth, he may recite "Amen" even if he has yet to swallow.
3) If one mistakenly spoke before swallowing, but after placing the food in his mouth, he need not repeat the Beracha; but if he spoke after reciting the Beracha before placing the food in his mouth, he must repeat the Beracha.
4) If one person recited the Beracha on behalf of others, as a host generally does at Shabbat meals, the guests need not recite the Beracha if the host speaks before partaking of the bread.
5) If the guests speak before partaking of their pieces of bread, then so long as they began speaking only after the host partook of his piece of bread, they need not recite a new Beracha.
