It’s a normal tendency of a human being to speak loudly which mostly depends on their audibility. People when speak also listen to others what they are speaking. We raise our voice a bit louder when we are not able to understand ourselves what we are speaking.
It’s in our habit to speak quietly if we are in a closed room or library etc. It’s not because we are discipline it is because it is in our habit. If we speak quietly in the closed room, then in places like markets, malls we tend to raise our voice.
Now, if we are hearing songs through earphones we tend to speak loudly because the sound of music dominates our voice. It is also because the eardrums receive music more loudly than our own voice and the sound of our voice gets obstructed by the earphones. We all have seen and noticed that even if we are putting our earphones and no music is being played, we tend to speak loudly as in case of cotton buds.
So, the loudness of our voice depends on how clear and audible we are to ourselves. When we are speaking with earphones put on we speak louder until our voice appears to be normal to us.