[thedoc (OP)]

Shoaib asked the Naked Scientists:
   why we respond louder than normal to people why listening music on headphone?
What do you think?

[evan_au]

We hear loud sounds in our earphones, so we assume the other person will, too. So we naturally speak more loudly.

The opposite effect occurs if we wear earplugs in an aeroplane. We can't hear the noise of the engines, so we assume the stewardess can't either. So we naturally speak more softly.

The solution is the same in both cases - remove the obstruction (headphones/earplugs/music), we will be in the same noise environment as the other person, and our natural volume control will work properly.