Gemara

Reuven was about to die (he was Muchzak not to have brothers). He said 'I do not have brothers.'

(Rav Yosef): We are not concerned (lest he has brothers). Firstly, he was Muchzak not to have brothers. Also, he said 'I do not have brothers'!

Abaye: People are saying that witnesses overseas know that he has brothers!

Rav Yosef: Since there are no witnesses in front of us we are not concerned, like R. Chanina taught:

(R. Chanina): (A woman told Beis Din that she was captured, but was not defiled. She is believed.) Will we forbid her because people are saying that there are witnesses overseas?!

Question (Abaye): We are lenient about a Shevuyah (a captured woman. She makes herself repulsive, so her captors will not desire her). We cannot learn to permit a woman to marry (perhaps she needs Chalitzah)!

Rava (to Rav Noson bar Ami): Be concerned for her.

Kidushin 12a: Reuven was Mekadesh Leah with a piece of marble. Rav Chisda was calculating whether it is worth a Perutah.

Rabanan (to Rav Chisda): There are witnesses abroad that the rock was worth a Perutah that day!

Rav Chisda: Since they are not here, we are not concerned for them.

Suggestion: This is like R. Chanina's ruling!

Rejection (Abaye and Rava): R. Chanina is lenient only about a captured woman. Here, if the Kol (rumor) is true, she is married to Reuven!

Leah married someone else. In Sura, Chachamim refused to intermarry with her children, because they hold like Abaye and Rava.

Rishonim

The Rif and Rosh (Bava Basra 8:39) bring our Gemara. (It seems that our Gemara's text is correct, that he was Muchzak not to have brothers.)

Nimukei Yosef (DH Im): We are more lenient about a Shevuyah for it is a lighter Isur, and even that Isur we need not be concerned for, for she guards herself from Zenus (illicit relations). Can we be lenient about an Eshes Ish, who until now was Muchzak to be forbidden due to a severe Isur?! Can we permit her due to Safek and overturn her Chazakah?! Rava admitted, and commanded not to permit her until we see if there are witnesses, and what they have to say. A Mishnah teaches that if a man said that he has sons; he is believed, for he could have permitted his wife through a Get. There is different, for there is no Kol. Here, there is a Kol that there are witnesses, so we must be concerned for them. We do not silence the Kol. The case is, we know that he has no sons. The Ramban says that we discuss a Kol that was established in Beis Din, for we hold (Gitin 89a) that if not, we need not be concerned for the Kol.

Rambam (Hilchos Yibum 3:3): If a man was not Muchzak to have brothers, and there was a Kol that there are witnesses overseas who will testify that he has brothers, even if he said at the time of death that he has no brothers, she is concerned and must wait until the mentioned witnesses come, and we ask them.

Magid Mishneh: The Ramban and Rashba say that we discuss a Kol that was established in Beis Din. Some do not require this. The Rambam holds like the latter opinion.

Tosfos (Kidushin 12b DH Im): We are more lenient about a Shevuyah, for even if there were witnesses that she was captured, the Torah permits her. She is forbidden only mid'Rabanan. Some say that a Shevuyah is more lenient for it is a lighter Isur (a Lav), whereas Eshes Ish has a Chiyuv Misah. This is wrong. Also a Shevuyah can entail a Chiyuv Misah, e.g. if she marries a Kohen, her son is a Chalal. If he will serve in the Mikdash on Shabbos, he will be Chayav Misah!

Poskim

Shulchan Aruch (EH 157:8): If a man was not Muchzak to have brothers, and there was a Kol that he has brothers and there are witnesses overseas, even if he said at the time of death that he has no brothers, she is concerned and must wait until the mentioned witnesses come, and we ask them.

Beis Shmuel (10): This is even if he said that he has no brothers before the Kol began. The Gemara connotes like this.

Ran (Kidushin 6b DH understand'Amrinan): We discuss a Kol that was not supported. If it was supported, Rav Chisda would agree that we are concerned. Abaye and Rava hold that even though normally we are not concerned for a Kol that was not supported, this is to keep the old Chazakah, e.g. a single girl and there is a Kol of Kidushin. If she accepted Kidushin, but we are unsure whether it was a Perutah, we are concerned even for an supported Kol. We are lenient about a Shevuyah. Similarly, we are concerned for a Kol that witnesses know that there are brothers.

Mahari Ben Lev (2:19, cited in Be'er Heitev 9): Since regarding Isur we are concerned for the witnesses, also regarding money (Beis Din does not give it to the heirs until the witnesses come and testify). How can the Ran compare the case of a stone to the Yevamah? In the former case, she accepted Kidushin, like the Ran himself distinguished! The Yevamah had Chezkas Heter. Only a supported Kol can forbid her! Rav Yosef said 'firstly, he was Muchzak not to have brothers.' This implies that without the Chazakah, we would forbid, even without a Kol! Regarding the stone, we are concerned only due to the Kol of witnesses, and Rav Chisda is not concerned even with a Kol! Since we know that she has no children, like the Rashbam says, she has Chezkas Isur, until we know that there are no brothers. Therefore, without a Chazakah we would forbid due to Safek, for it is common to have brothers. If a stone is not worth a Perutah today, it was probably not worth a Perutah at the time of Kidushin. We do not forbid due to such a Safek (by itself). The Tur forbids only when there is a Kol about witnesses. He could agree with the Rashbam that she is forbidden even without a Kol when we know that there are no children. The Tur discusses when merely he is not Muchzak to have children. The Rivash says that we do not give inheritance if there is a Safek of relatives elsewhere, unless we are Muchzak that there are no closer relatives elsewhere. However, Teshuvah ha'Rosh says that we give, unless we are Muchzak that there is a closer relative elsewhere.

Mahari Veil (88): The Rambam holds that one witness is believed even to say that the Yavam died, against the Chazakah. The Rosh says that it is difficult to permit a Yevamah through one witness, i.e. against Chezkas Isur, but he agrees that one witness is believed (to say that her son is still alive) to support the Chazakah. Even if there is a Kol that he died, the witness is believed to silence the Kol. If we know that Reuven's father was a Kohen, and there was a Kol that Reuven's mother was a divorcee or Chalutzah, we demote him (from Kehunah). If one witness testified that he is a (Kosher) Kohen, we reinstate him (Kesuvos 26a). This shows that the Torah believes one witness against a Kol. Perhaps one will say that we suspect lest the witness tries to ruin her. When a witness says that first her husband died and then her children died, Tosfos and the Rosh say that we are concerned lest the children died first, and the witness seeks to ruin her. This is only because he did not need to testify about her children's death to permit her. Normally, we are not concerned. If not, we would not believe one witness to permit an Eshes Ish or a Yevamah to remarry! Even a child who became an apostate is believed to give her (initial Yisrael) name and her mother's name to permit her mother. This is like the man who said 'I, Ploni ben Almoni, was bitten by a snake, and I am dying.' They checked and did not recognize him, and Chachamim permitted his wife to remarry.

Tosfos (135a DH Ha): The Kol says that there are witnesses overseas. The text does not say 'there are witnesses overseas', for if we know that there are witnesses, why do we say 'now, there are no witnesses'? If there is no Kol, we are not concerned, for we permit a woman who says 'I was captured, but I was not defiled.'

Mahari Veil (88): Therefore, we are not concerned for a Kol that her husband said that it is not his daughter, or that he was not with his wife for the 12 months before she was born, if the Kol does not mention witnesses. The Rashbam says that we must wait until we hear what the witnesses say. This implies that the Kol says who the witnesses are. Also the Rambam connotes like this. He connotes that the witnesses already testified. If there is evidence that the Yavam himself started the Kol, we are not concerned. Even when there are enemies we are not concerned for a Kol, and all the more so if the Yavam started the Kol. We are concerned for a Kol that there are witnesses when it is likely that witnesses saw what is needed, e.g. that a man has brothers, that a stone was worth a Perutah, or that a woman (who admits that she had relations) was not forced. However, it is unlikely to find witnesses of Zenus or something very close to it. Normally, a woman who has Zenus does so in private. Tosfos (Kidushin 12b) distinguished between when the witnesses need to testify about one thing or more than one (i.e. testimony of captivity does not forbid mid'Oraisa). One witness is believed to silence a Kol about Kashrus of a Kohen, even without a reason to support him. Therefore, he is believed to silence a Kol about a Yevamah, even if we cannot say that she will be careful to check. We can learn a Kal va'Chomer. One witness can silence a Kol regarding Chayavei Krisos, and all the more so for a Lav!