I cannot tell from your question if you're concern is focused on Kol Isha. I am assuming that is the focus. This article discusses Kol Isha and when and how it applies, citing various viewpoints. I am citing one of the permissible views. Read the remaining sections of the article, as there are varying opinions.
Rabbi Yehiel Yaakov Weinberg (Seridei Esh 2:8) defended the
German-Jewish custom of mixed-sex zemirot, as well mixed sex Jewish
youth groups (similar to Benei Akiva and NCSY). He interpreted the
Rambam that the prohibited form of "gazing" and listening is only that
which entails sexual pleasure. Rabbi Weinberg also relied on the Sedei
Hemed (section Kuf, kelal 42), who ruled that a man's listening to a
woman sing funeral dirges does not violate kol ishah, as no sexual
pleasure is entailed in such songs. The Sedei Hemed was relying on the
Divrei Heifetz (113b), who "stated that as long as a woman is not
singing sensual love songs, and as long as a man does not intend to
derive pleasure from her voice, there is no prohibition, such as if
she is singing praises to God for a miracle, or is singing a lullaby
to a baby, or is wailing at a funeral."  Rabbi Weinberg also cited
the Sefer ha-Eshkol (Hilkhot Tefillah sec. 4 or 7), that listening to
a woman sing is prohibited only where there is sexual pleasure. Rabbi
Weinberg reasoned that if the Sedei Hemed could permit funeral dirges
due to their lacking sexual pleasure, then he could permit Shabbat
zemirot on the same grounds. It is obvious that we today can likewise
permit by the same logic any song which does not lead to sexual
thoughts. Thus, this interpretation that kol ishah is like etzba
ketana, i.e. permitted where sexual pleasure is absent, is not only
apparent from the simple meaning of Rambam's words, but is also
endorsed by Rabbi Yehiel Weinberg.
Rabbi Weinberg also relied on the opinion of the Ritva and Rema, "that
all is for the sake of heaven" (to be discussed later in this essay).
In general, if one knows that he himself is capable of a certain act
without incurring sexual thoughts, then this act becomes permitted for
I am inferring from above as well as the rest of the article, that the main concerns are related to singing. As you can see, there are leniencies, there. I think that hearing a woman speak is less of a concern of Kol Isha.
If your concern is regarding mixed seating, I can say from my personal experience, that I have attended numerous services where there was a mechitza for davening. A woman gave the D'var Torah before Musaph on Shabbat, and she went to the front of the shul, and the mechitzah curtain was drawn so that the women could see her. It was a given, then, that the men could see her as well as the other women.