I say it Air-a-gorn.... maybe it's British pronunciation?
Hmmm...I kinda like it. It seems to me that it's a more "proper" way of pronouncing it and even The Dark Lord gives a sort of homage to Aragorn's lineage by saying it that way. Maybe PJ did it that way to show the difference between the common tongue and high speech, because Arwen says it to Aragorn while she's speaking elvish..
I have no idea, but I like it!