Teshuvos Achronim continued.
Chasam Sofer V:CM2 (the V is a Roman numeral and the CM is not - it's Volume 5: Choshen Mishpat 2)
The Dayan Rav Daniel made a diyuk in Rambam Eidus end of Ch. 9 and and SA CM 35:12 that "a blind person cannot testify even if he recognizes the voice" implies a kosher witness can testify when he recognizes the voice. But Chasam Sofer goes into the issue further and suggests maybe kol only works with touching the person also, like Yitzchak felt Yaakov and felt hairy Eisav-like arms. He concludes we would not be motzi mammon based on recognizing a voice.
(2 non-post-related points of interest: the first is he discusses standing in dinei Torah (in the legal sense - you can't sue someone if they have no standing; he mentions "lav baal devarim didi at." Minor point.
The second is timely - the case was where a person hid witnesses and told them he was going to bring a certain person (let's call him Reuvein) in to admit Reuvein owes this guy money. Chasam Sofer says even if tevias ayin of kol wirks, in that case it would not work because the witnesses are tainted thiking the voice is Ruvein's. But maybe the guy brought in someone to imitate Reuvein's voice. This is timely because of Daniel Kahneman's recent book Thinking, Fast and Slow discusses the effects of priming - putting an idea or awareness of something into a subject's consciousness - even non-maliciously -and the subject's answer can change by putting a certain idea into his head. (Enjoy that, #JR!)
Mishkenos Yaakov in Kehilas Yaakov (there are teshuvos in the back of it) EH 6. Discusses using kol in aguna. Cites Avodas Hagershuni, Shvus Yaakov and Ketzos. Leans toward saying like Ketzos. He keeps referring to another Teshuva - not sure if it is from a question that is not printed or is in R' Efrayim Zalman Margolis' teshuva (maybe Beis Efrayim EH 15-19?) about making a machlokes Bavli/Yerushalmi of the status of siman being dioraissa or derabanan. I lost the thread of his discussion because I don't know what he keeps referring to. He does say a chiddush that while it sounds like we rely on teviyas ayin in returning an aveida to a tzurva dirabanan, he says it is not because we rely on Tevias Ayin, it is because in that case, the owner had yiush.
Somewhere (can't remember) I was referred to Noda BiYehuda Kama EH 51 but it does not seem to e about kol.
And I want to end with how I satrted this series - if Eidim were hidden and heard someone admit to a debt, it is considered admitting to his debt, the Ketzos says it is only where the witnesses were able to see the face of the admitting person, and the Nesivos says no, we rely on the witnesses identifying the admitting person by recognizing his voice.
Applications of this issue
This issue comes up several times in the latter half of sefer Breishis:
1) When Racdhel gave the signs to Leah (whatever they were) and Lavan successfully swindled Yaakov into marrying Leah. Why didn't Yaakov recognize that the voice was not Leah's?
2) Hakol Kol Yaakov - when Yaakov got the brachos from his blind father, who thought it was Eisav. Why didn't Yitzchak identify Yaakov by voice?
3) It seems one reason why Yosef spoke harshly to his brothers when they came for food was so they would not recognize his voice. If he would have spoken regularly they could have identified him.
These examples are mentioned at some point by several of the teshuvos mentioned above.