I think when I'm 'being myself' around people I'm comfortable with, I speak louder, and when I'm talking to strangers or people I'm not comfortable with I tend to speak quietly (sometimes to the point where they don't realise I've even spoken).
For example, I have been told I speak loudly by my family and my partner. My partner is always telling me to lower my voice when we eat out because other people can hear our personal conversation. Another example is my first job at a supermarket checkout, where customers would regularly not hear me when I greeted them. One lady called me rude to my face for not greeting her, and marched off to complain to my supervisor! I had greeted her, just too quietly.
On the other hand, maybe it's my environment as well? I have Central Auditory Processing Disorder so I tend to miss bits and pieces when people talk to me. Add loud background noise, like a restaurant, and I struggle even more, so I must automatically speak louder assuming the other person has the same increased difficulty I do.