As previously cited1, the Talmud Bavli (Masekhet Berakhot 24A) refers to a woman's voice as her nakedness (based on Shir HaShirim 2:14).
If a woman's voice is her nakedness, why would the man speaking in the verse (2:14) ask to hear the voice of the woman being referred to?
יוֹנָתִי בְּחַגְוֵי הַסֶּלַע בְּסֵתֶר הַמַּדְרֵגָה הַרְאִינִי אֶת מַרְאַיִךְ הַשְׁמִיעִנִי אֶת קוֹלֵךְ כִּי קוֹלֵךְ עָרֵב וּמַרְאֵיךְ נָאוֶה:
My dove, in the clefts of the rock, in the coverture of the steps, show me your appearance, let me hear your voice, for your voice is pleasant and your appearance is comely.'
1. Technically, the first citation I found was HaRav Hayim (Howard) Jachter's The Parameters of Kol Isha.