This post is not about Kiddushin 80 - kol b'isha erva, but we will end up there in this series.
Parshas Vayeitzei 29:25, see Rashi.
Bava Basra 123a says Rachel gave the simanim to Leah, and Eicha [Rabasi] Pesichta 24 says Rachel hid under the bed and gave them to Yaakov. What is the difference between the two midrashim? Sanhedrin 38 says everyone is unique in three ways: appearance, voice and thoughts. Appearance and voice for Erva purposes - so you can recognize your spouse. Therefore the midrash said Rachel hid under the bed so it was her voice - which Yaakov knew - answering Yaakov with the simanim. What would the Gemara Bava Basra (that Rachel told the simanim to Leah, so Leah gave them to Yaakov) do with the Gemara in Sanhedrin - Yaakov should have realized the voice did not belong to Rachel?
Maybe, as siblings, Rachel and Leah had similar voices and could not be distinguished accurately. Maybe that they didn't have too much to do with each other until the marriage so they didn't know each other's voices too well (Torah Shlayma Breishis 29:25:84 cites this possibility in the names of Ri miVina, Riva in Drashas ibn Shavib and Baalei HaTosfos). And maybe the siman was not verbal, but was an amulet (Torah Shlayma brings such a possibility) so voice never came into play. And see Maharsha, Bava Basra 123a s.v. Vehaynu.