The Gemara (Berachot 24a) states, “The voice of a woman is Ervah, as the Pasuk [in Shir Hashirim 2:14] states ‘let me hear your voice because your voice is pleasant and appearance attractive.’” Rashi explains that the Pasuk in Shir Hashirim indicates that a woman’s voice is attractive to a man, and is thus prohibited to him. Rav Hai Gaon (cited in the Mordechai, Berachot 80) writes that this restriction applies to a man who is reading Kriat Shema, because a woman’s singing will distract him. The Rosh (Berachot 3:37) disagrees and writes that the Gemara refers to all situations and is not limited to Kriat Shema. The Shulchan Aruch rules that the Kol Isha restriction applies to both Kriat Shema (Orach Chaim 75:3) and other contexts (Even Haezer 21:2). The Rama (O.C. 75:3) and Bait Shmuel (21:4) clarify that this prohibition applies only to a woman’s singing voice and not to her speaking voice.
How can you rule on Kol Isha based on Shir HaShirim. Shir HaShirim is not a love story between a man and a woman. It is a parable about G-d and the Jewish People.
Also, kol isha is a chumra, not actual Halacha. The only prohibition of Kol Isha is during prayer. Mixed seating and women leading synagogue services are forbidden.