Cut a longitudinal slot in the side of a loo roll holder in which the iPhone will just stand.
Call the slot side the top, and push four map pins into the bottom to make legs.
Stick iPhone into slot and turn on.
Result: sound that is unbelievably better than the same phone out in the open.
Try it, it is an amazing difference.
It turns out there is a commercial passive iPhone4/4S sound improver too.
From WH Smiths, as part of its GadgetShop range, this is a rubbery thing with an exponential horn that slides onto the bottom of the phone.
The phone has two sound ports on the bottom, and the rubber thing looks like it picks up both, which made me think there would be a rather odd effect on the spectrum as one port is half way along the horn.
However, it actually only connects to the one at the end of the horn, and blanks off the other.