I don't even see what the difference is whether they film outside or in a studio, or do the audio track seperately.
A hostage threat is a hostage threat. period.
But remember there's two different types of people crying "fake" here. At one end you've got people who are just saying ISIS might have edited it for effect, like combined two videos of real hostages into one. At the other end you've got people who say it was entirely faked by a mysterious world elite who control everything, and that ISIS does not even exist.