There are quite a few aftermarket accessories for the iPad that allow us to increase the sound naturally - a rather curious bamboo tube comes to mind - but perhaps the most aesthetic I have seen so far is SoundJaw.
Created by Matt McLachlan and funded one more time by Kickstarter, this accessory increases the sound power of the iPad 2 completely naturally by creating a curve that amplifies the sound.
The result is an extra 4 dB at maximum volume based on TUAW, a figure not negligible, and you have to remember that decibels are measured logarithmically.
Link | Soundjaw