In keeping with the small size of the boxes, there is a small-size
solution for this.  It will take a bit of ingenuity and help from
a knowledgeable Circuit City employee or the like but this will do
the job beautifully: a pair of self-amplified computer speakers.

Some fairly small size ones do quite an amazing job of room-filling.
Use a small microphone with the right impedance not to overdrive the
speaker input.  It may even be possible to use a contact mike such
as is used with violins and other instruments.  It picks up vibrations
in the wood of the box more than through the air.  The whole setup
could fit in a handbag or two coat pockets!

Jim Katz