Gemara

(Beraisa): Mesis (one who entices to serve idolatry) is the only capital transgression in which witnesses hide in order to trap someone. We light a candle an inner room, and witnesses are in the outer room. They can see and hear the Mesis, but he cannot see them.

Chulin 96a (Rava): I used to think that a Siman is better than Tevi'as Ayin (recognition), for we return a lost object to anyone based on a Siman, but only to a Chacham through Tevi'as Ayin. I retract due to these teachings which show that Tevi'as Ayin is no worse (it permitted meat and Techeles).

Support #1: If we would not rely on Tevi'as Ayin of voice, why may a Suma (blind person) have Bi'ah with his wife (perhaps it is another man's wife)? Why may anyone have Bi'ah at night (when he cannot see)?

Support #2 (R. Yitzchak brei d'Rav Mesharshiya): If two witnesses do not recognize the murderer but they know Simanim on his body and clothing, we do not kill him. If they recognize him, we kill him.

Bava Metzia 20a: A Get was found in Beis Din. Rabah proved that we are not concerned lest it is a different Get, for a Mishnah says that we return all documents of actions of Beis Din.

Question (Rav Amram): How can you learn about a Get, which affects an Isur, from monetary laws?

Shevuos 33b (Beraisa - R. Yosi ha'Galili): "V'Hu Ed Oh Ro'oh Oh Yoda" discusses (monetary) testimony that is possible through seeing without knowing, or vice-versa, e.g. they heard Shimon admit to Reuven.

34a (Abaye): R. Yosi ha'Galili could agree with R. Acha (who says that if a dead camel was found near a kicking (or mating) camel, surely, the latter killed it. Indeed, he would accept such testimony also in capital cases, like the case of R. Shimon ben Shetach (he saw Reuven chase Levi into a ruin, and ran after him, and saw Levi dying and blood dripping from a knife in Reuven's hand). However, seeing without knowing does not help in capital cases.

We must say that they saw a human form with a shadow of a shadow. If not, we would be concerned lest it was a Shed (demon).

Question: Perhaps it was a Tzarah (intending that her Tzarah remarry and later be forced to leave her husband)!

Answer (Tana d'Vei R. Yishmael): At a time of danger, we may write and give a Get even if we do not recognize the husband (the same applies here.)

Rishonim

Rif (Chulin 34a): The Gemara proves that Tevi'as Ayin is better than Simanim. If not, a blind man, or anyone at night, would be forbidden to have Bi'ah!

Rambam (Hilchos Edus 9:12): The Torah disqualifies a Suma for testimony, even if he recognizes voices and knows people - "v'Hu Ed Oh Ro'oh Oh Yoda."

Shiltei ha'Giborim (Sanhedrin 8a, b'Sof): The Ramah learns from 67a that witnesses can testify about someone only if they see him and hear his voice.

Poskim

Shulchan Aruch (CM 35:12): If one is blind in both eyes, even if he recognizes people's voices and gives proper testimony, it is Pasul.

Beis Yosef (DH Suma): We say in Gitin that even if he recognizes through Tevi'as Kala, he is Pasul. Rashi (Nidah Sof 49b) says that "Oh Ro'oh" disqualifies him from testimony.

Prishah (14): Our text of the Tur disqualifies a Suma even if he recognizes the voice and the people. Some texts say 'the voice of the people.' The Rambam writes 'even if he recognizes the voice and knows the people.'

Shulchan Aruch (81:13): If one hid witnesses in a Kilah (a bed with curtains over it), and Ploni admitted that he owes money and consented that the waking and sleeping be witnesses, it is a valid admission.

Ketzos ha'Choshen (13): It is not clear whether the witnesses must see and recognize Ploni, or if Tevi'as Kala suffices. Chulin 96a connotes that Tevi'as Kala suffices even for capital cases. However, in Sanhedrin 67a we say that hidden witnesses do not help for capital cases, except for a Mesis. We light a lamp in the inner room, and the witnesses see him from the outer room. Rashi says that even though they hear him, they must see him. If not, he could say that it was someone else. This connotes that Tevi'as Kala does not help for capital cases, and even for monetary cases to make someone pay. The Pri Chodosh says that Rashi equates Tevi'as Kala to Tevi'as Ayin only for Isur, to permit a Suma (or any man at night) to his wife. Alternatively, we do not rely on Tevi'as Kala for a Mesis, for presumably he speaks in a whisper, unlike his usual voice; this is primary. I say that his first answer is primary. The Ramah learned from Mesis that witnesses must see the person and hear his voice. According to the Pri Chodosh, we cannot learn from Mesis! Rather, Tevi'as Kala helps only for Isur, for which even Simanim work. We do not rely on Simanim to make someone pay (Tosfos Chulin 96a DH Planya).

Rebuttal (Nesivos ha'Mishpat Bi'urim 7): We say (Bava Metzia 20b) regarding Simanim that Isur is more stringent than money. Yosef ben Shimon is a valid Siman for money (we are not concerned lest someone else has this name), but not for Isur. Since Tevi'as Kala works for Isur, all the more so it works for money! Capital cases are different, for they require seeing and knowledge. For monetary cases, it is enough to have knowledge without seeing (CM 30:14).

Defense (Meshovev Nesivos 7): Knowledge without seeing is like the case of R. Shimon ben Shetach, for he deduced what happened without seeing the event. If Tevi'as Kala helps, it is called seeing, for it does not depend on deduction. (The same applies to anything the senses perceive.) Rather, we must say that Tevi'as Kala is too weak, so we do not rely on it, and the same applies to monetary cases. Simanim (for Aveidos) are different.

Chasam Sofer (CM 2): The Ramban (Bereishis 27:12) says that Yakov was not concerned lest Yitzchak recognize his voice, for perhaps the brothers had similar voices, or he could mimic Esav's voice; some people can do so. ("Ha'Kol Kol Yakov" refers to a soft language and mentioning Hash-m.) If people can mimic others' voices, we may rely on Tevi'as Kala only Lefi Tumo (normally), but not when someone hid witnesses, lest someone else mimic the voice of the 'borrower'. Our text of the Rambam disqualifies a Suma even if he recognizes voices or knows people, i.e. but a Pike'ach (one who can see) may testify based on Tevi'as Kala. However, the Prishah's text of the Rambam is 'he recognizes the voice and knows the people. The Chidush is that even in this case a Suma may not testify. We may not infer that a Pike'ach needs both to testify, nor can we infer that voice alone suffices. Perhaps he must also feel, like Yitzchak felt Yakov. The source to rely on Tevi'as Kala is the Heter of one's wife when he cannot see her. Perhaps we also need recognition of the body! If Tevi'as Kala alone suffices, why must witnesses see a Mesis? Also, in Shevuos, perhaps "Oh Ro'oh Oh Yoda" helps for capital cases for Tevi'as Kala of witness who can see! Rather, it was clear to the Gemara that Tevi'as Kala is worse than the cases of R. Acha and R. Shimon ben Shetach. Even Rav Acha would not say that we kill due to Tevi'as Kala. Since the Halachah does not follow R.Acha even for monetary cases, we do not make one pay due to Tevi'as Kala unless it is accompanied by other proofs better than seeing and knowledge. If one witness says (that a man died) by estimation, we do not transfer money due to him. All the more so we do not transfer money due to two witnesses of Tevi'as Kala. Especially if the witness expects Ploni to come, he thinks that it is Ploni's voice.

Chasam Sofer (Gitin 23a DH ha'Kol): Seemingly, since a person could know testimony without seeing, just a Gezeras ha'Kasuv disqualifies, we disqualify only a Suma, but a Pike'ach can testify based on Tevi'as Kala. However, Rashi (Sanhedrin 67a) holds that it is not a Gezeras ha'Kasuv. Even a Pike'ach must see the testimony, for if not, the defendant could say 'it was not me.' This is difficult, for we concluded that Tevi'as Kala is full recognition! Rather, this must refer to Tevi'as Kala with feeling.

Chasam Sofer (CM 2): Keneses ha'Gedolah says that the Rambam learns from the need for "Oh Ro'oh." "Oh Yoda" was not enough, for without seeing this is not knowledge. This is astounding! The Gemara says that it is needed for seeing without knowledge! Rather, the Rambam expounds the simple meaning (not extra words).