When I was a boy davening in shul, none of the men was ever really silent. They just davened their way through the amidah as fast as they could, to see who could finish first. I never could keep up. The opening was composed about 2500 years ago, and with the exception of the Shema- is probably the oldest fixed daily prayer in our liturgy. Since it was customary for sinners to confess their sins during the course of worship, reciting prayers aloud would force all confessions to be public.
Silent prayer avoids public shame for one's sins.
But the most popular explanation offered for quiet or silent prayer is that to the Rabbis of the Talmud prayer is a service of the heart, and God knows what goes on in a person's heart without him shouting it. Many congregants could not read, so it was determined that the Cantor would recite the prayer aloud, and the congregants would say amen after each blessing and thus fulfill their obligation. I ' m not a regular at Friday night services at Larchmont Temple, but when I attended this past Friday night I was puzzled at how the Amidah is done. And I also thought that the Amidah was a silent prayer. We sit down in the middle, we sing part of it, and the prayerbook doesn't even mention the word Amidah.