Science + Son has developed two generations (I and II) of these patent-pending Phonophone passive amplification speakers. From the website:
Through passive amplification alone, These unique pieces instantly transform any personal music player + earbuds into a sculptural audio console.
Without the use of external power or batteries, the Phonofone inventively exploits the virtues of horn acoustics to boost the audio output of standard earphones to up to 55 decibels (or roughly the maximum volume of laptop speakers).
Upon connecting active earphones to the Phonofone their trebly buzzing is instantly and profoundly transformed into a warm, rich and resonant sound.
The Phonofone is constructed entirely from ceramic. Not only environmentally low impact, ceramics are inherently rigid and resonant, lending themselves well to this application.
This diagram shows a little more about what is going on, acoustically speaking.
The pictures may be a little deceiving: The actual size is 11″ x 9.5″ x 20″. Unfortunately, the price is going to run a little more than your iPod or iPhone. Charles and Marie are selling a Phononphone II online for $500.