In the amazing letter written by the Vilna Gaon, he writes:
But the main way to merit Olam Haba is by guarding one's tongue. That is worth more than all the Torah and good deeds. This is the meaning of (Isaiah 32:9) "tranquil women" (Berachos 17a) because the mouth is the holiest of the holy.
How does “tranquil women” relate to the mouth being holy?