I hardly ever use the earplugs/headset, but I think on most planes it is the dual mono plugs. I think it was when I flew Air Gabon (Air NoBag), they had the dual mono plugs, but when you were finished flying you could cut off the extra plug and remove some plastic from the remaining plug and voila, you had a normal 1/8" stereoplug for your walkman/discman/whatever.

None of the times I have flown internationally, did we have to pay for headsets/earplugs. And this was in cattle class.

Stig