For much of the Church’s history many of the priest’s prayers at Mass were inaudible to the congregation. This wasn’t simply because they didn’t have microphones or because the priest prayed ad orientem, facing in the same direction as the congregation. It was the style of the liturgy, and you will still experience the Mass this way if you attend the Extraordinary Form.
Even today, however, in the post-conciliar Mass, during which the priest generally faces the people and many parts of the liturgy are prayed responsively or collectively, there are still some prayers of the priest that those outside the sanctuary may not be able to hear
Why is that?
According to Fr. Edward McNamara, “In the ordinary form of the Roman Rite this quiet recitation is mostly reserved to the priest’s personal prayers.” Sometimes these prayers are called “‘priestly apologies,'” which are not prayers in which the celebrant excuses himself for being a priest, but in which he recognizes his intrinsic indignity and implores divine aid in order to worthily celebrate the august mysteries.”