I've noticed that people go on of two ways when they have headphones on.

Tries to compensate for the volume, therefore being a bit too focus on themselves - naturally assuming that everyone hears what they hear. Therefore, has to shout in order to be heard.

Thinking no one can hear them, since the volume's too loud therefore miming all of their words out. Furthermore, it could just be a lack of volume control.

Personally, I'm a Shouter even without headphones on...

However, the other day on the bus I met a Mimer, he was saying thank you to me as I moved my bag so he could sit down and with headphones on he mimed 'thank you'