d00d ! (ahem)
B&W are arch-rivals of this idea with cone rigidity being one of their main tenants.
I think that any material that changes with humidity could be an issue.
My wife's car stereo sounds like ass when it's rainy with her factory paper cone speakers.
Hemp-cone speakers would have to be treated to not absorb moisture. It's basically a just type of paper.
I would guess that a bit of marketing is afoot. Any plant-based material made into thin sheets is generically paper, with a lot of marsh grasses and other renewable resources being used today.
Hmm, I wonder if Cypress Hill sounds better on them