First, that video is post processed with some kind of filter, that give it that nice sepia tone and I think also some lighting effects as well has been added in post.
Also, it's not made in real time, FSX has been set to "full sliders to the right" for everything, the simulation rate is set to 1/4x and the video is then accelerated to 4x.
The audio, of course, doesn't come from FSX, but it's a real world in-cockpit recording, which matches nicely to the flight sim scenery...
This was explained on the original post on the italian forum that linked to the youtube video page: the author of the video never hide the fact the video is not a real-time recording.
But it's a great video nonetheless, and shows a good craftmanship in video editing.