When we speak we are not only speaking but also listening to our own voice, hence we keep the volume of voice at suitable hearing level. It means that if the volume is adequate for self-hearing we assumes that it is adequate for the people we are speaking to. As a result we keep the volume of our voice at the level we think is adequate. This has become instinctive and natural habit so one does not have to make conscious effort. In the noisy environment of a crowded restaurant everybody speaks somewhat louder because one starts finding one’s own normal volume rather inadequate. Short sentences in hushed tones falling on the ears in the library are not as much due to discipline as the silence pervading in the reading room. In short, the speaker adjusts volume of his speech to the volume of sound falling on his ears.
Since headphones covering the ears (even if music is not playing) obstruct hearing, one unconsciously utters loudly.