What is the status of eidus based on identifying someone by their voice?
If this does work, is that only when the "witness" recognizes the voice? What if the witness does not recognize the voice, but the voice identifies itself?
Let's look at various teshuvos about this.
Rishon - Ri Migash 149
Identifying from voice works. The shaila was for mekalel or menadeh, so it seems if you heard someone be mevarech es Hashem you can give a chiyuv misa, provided there was a hasraah.
Interesting Gemara (cited by Ri Migash): Gittin 66a
תלמוד בבלי מסכת גיטין דף סו עמוד א
מתני'. מי שהיה מושלך לבור, ואמר כל השומע את קולו יכתוב גט לאשתו - הרי אלו יכתבו ויתנו.
גמ'. וליחוש שמא שד הוא! א"ר יהודה: כשראו לו דמות אדם. אינהו נמי אידמויי אידמו! דחזו ליה בבואה. אינהו נמי אית להו בבואה! דחזו ליה בבואה דבבואה. ודלמא אינהו נמי אית להו! א"ר חנינא, לימדני יונתן בני: בבואה אית להו, בבואה דבבואה לית להו. ודלמא צרה היא! תנא דבי רבי ישמעאל: בשעת הסכנה, כותבין ונותנין אע"פ שאין מכירין.
The Rambam (Geirushin 2:13) does not bring down about בבואה דבבואה; see Kesef Mishna. (My Rebbi discussed the "sheid" aspect in Hashkafa shiur here.)
Avodas Hagershuni 110
The case was where a person had a brother abroad, but no one knew where. Someone came claiming to be the brother, and he wanted to inherit their father's estate with his brother. No one recognized his face, but people did recognize his voice. Is that enough? Avodas Hagershuni says recognizing the voice is enough even in dinei mammonos, provided there is no contradictory evidence. Therefore, in this case, there are eidim who recognize his voice, however, his identity is in doubt because everybody agrees his face does not look like the brother we knew before he went abroad, so he cannot inherit absent better proof that he is who he claims he is.
Applying this to Gittin 66, Kol is a siman. When there is no other evidence, it works. When the gemara says "dilma sheid hu," it means how are you so sure it is a person and not a ventriloquist or Frank Caliendo imitating a voice or people trying to play tricks? בבואה דבבואה tells us there is no evidence contrary - call it reiusa or hakchasha - against this voice.
Shvus Yaakov I:100
The case was to matir an aguna. During a war or rebellion her husband went missing. Witnesses came that they heard rebels catch someone and that man was pleading for his life, and from what he was saying they knew he was this woman's husband. However, they were too far away to visually identify him, and it was very dangerous for them to try to get closer. Is their testimony of identifying this man by his voice enough to allow this aguna to remarry?
He says we don't have Halachik precedent regarding this. He cites the Avodas Hagershuni. He goes through the sugyos relating to it (and says a suma or even a seeing and hearing person is allwed to their wife even on their first night of marriage, based on recognizing their voice) and says we would allow it. Not for dinei nefashos, but certainly for this case and probably even for dinei mammonos. However, due to the novelty of the ruling, his heter allowing the wife to remarry would only take effect if two other great poskim agree to his heter. (This method of using three ballei horaah to be matir an aguna was used widely; even Igros Moshe has cases of it).
Comment: here we see that recognizing a voice does not mean that we know the person and recognize his voice, but even if we don't know the person but from what he says we can identify him - probably in this case, when the man was begging for his life, he probably said he had a wife and kids and probably mentioned their names, so the eidim who heard it were able to put the pieces together. That's how I understand the teshuva. This relates to the question I asked at the very beginning. It is important because Rashi to Gittin 66a says the voice identified itself, his wife's name and his city; the information necessary to write a get. He doesn't say if you recognize the voice or not; I think he means when you don't recognize the voice.