When the priest needed to be heard he said or sung the prayers in a loud audible tone. If you look also at the architecture of old church buildings they echo whenever you speak. They are naturally made to amplify the sounds. No microphone is needed.
When the priest’s prayers were not supposed to be heard by the congregation. During Solemn Mass the priest reads the prayers at the foot of the altar in a low inaudible tone- not able to be heard beyond the altar steps because the Schola is chanting the Introit at that time- he also prays the Introit himself in a low tone as well as the Kyrie- because again the Schola is singing that…also the Roman Canon is always prayed silently- no one can hear that because of its sacredness. There are also many other prayers that are intentionally prayed in a low inaudible voice because they are not intended to be heard by the congregation.
People did not do their private devotions or pray the rosary becaused they could not “actively participate” or couldn’t hear the preist. Some still do that today out of choice. As one who has attended the TLM when the priest would not wear a microphone- I sat in the back of the church and could hear him clearly whenever he prayed things that were supposed to be heard by the congregation.