Ylvis

Ylvis' song "What Does the Fox Say?" may be a constant assault on your ears -- but one good thing will come of it ... all the sexy fox costumes you're gonna see on Halloween! Turn it up, Ylvis!

There are going to be MILLIONS of foxes running around this Halloween ... 'cause the furry animal costumes are selling at record numbers this year -- and it's all thanks to that insane viral video "What Does the Fox Say," TMZ has learned. By now, you're probably one of the 131 MILLION people who…