Listen to the drums, boy
Now the saying "I hear it inside my head" will be something more real that just a psychological expression since the company Etrema have made a special metal solution that makes it possible to hear music inside your head without irritating others. In the notice Turn Your Head Into Speakers one guy is testing the new way of listening and his experience is direct: "It feels like the loud crunch that fills my head when I bite into a tortilla chip, except the crunch is music. The stereo sound is inescapable."
The main usefulness seem to be with sonar systems and alike military use but one could tell that it would be possible to use this technique to revolutionize the ear-phone-industry.