However, as another question, it would be interesting to know exactly how the sotto voce liturgical prayer developed.
* Fr. Jungmann (Vol 11, Pages 90-91)
Like them, it is spoken in the prayer posture of the orantes, and was likewise at one time (as is self evident) pronounced in a loud voice. Even today the final words Per omnia saceula saeculorum, like the Orantes at the start, which belong to it, are sung aloud...The first point to clear up is the puzzling problem of how the oratio super oblata came to be said silently. The earliest evidence of the quiet recitation of this prayer appears in the middle of the eighth century in Frankish territory... We then find we are forced to a second conclusion, that it was in Gallic territory that this low speaking was first employed for the Roman Oratio super oblata, just as was the case somewhat later in regard to the canon. For this low pronouncement of a liturgical text is as much in contradiction to ancient Roman usage as it is in harmony with the tendency of the Gallo-Frankish liturgy.