I don't have any particular insight, but I think it is a very interesting question, and I hope someone with knowledge will come forward, or maybe one of us will find a good source on the matter (I'll be busy today, but maybe I'll find time later)....
So I re-pose my question, originally posed to @marcusl, to you, @NTL2009: what do you think about the newer speakers that use neodymium magnets? I think rare earth magnets and mylar cones and high-precision tolerances in the manufacturing processes make for smaller speakers that are more robust and deliver great sound -- what do you think about those things? ...
But I'll take a semi- educated stab at it, and I'd say it's been an evolution, not a revolution. I think most would agree that sound quality in these small devices has generally improved over the past decade. I'd attribute it to mostly small improvements in materials, design, and also more attention to detail,and consumer demand. Early on, people were
Materials play a role, probably more computer modelling, and design-wise, I suspect they have been more clever in placing the speaker(s) and using the empty space in the device to provide a resonant cavity (like the box/port of a standard bookshelf speaker does).