Want speakers that are even more compact? Why not just build
them when you need them? NYC Japanese importer Compact-Impact has US$45
collapsible build-on-demand speakers
that measure just 3" cube and weigh only 225g, thanks to popypropylene
and, you know, cardboard. They're passive, 2 watts (that's right, two) so don't expect much sound. But maybe you should consider making miniature mixes just for them?