My favorite part is the flurry of media that has been created around what the fox actually DOES say. (Wired, Huffington Post (inclluding its ridiculous comedic remix), and a fabulous article by Popular Science)
In Norway, where Ylvis is from, there are two species of fox: the arctic fox (Vulpes lagopus) and the red fox (Vulpes vulpes). Here in the States, we have a few others, like the gray fox and the kit fox. All species of fox have a pretty wide variety of vocalizations, just as dogs and cats do. It's simple to reduce, say, a dog's vocalizations to "bark," but as any owner knows, dogs can yelp, whine, howl, growl, and make all kinds of other sounds. Foxes aren't quite as varied in their vocalizations as dogs, but they're still capable of making lots of different sounds. [PopSci]
But, in reality, most of us are happy to hypothesize along with Ylvis.