Update
This method for downloading youtube videos is now no longer effective since youtube streams both audio and video files seperately. However, you can use the same method below to download the video and audio file seperately and merge them using your favourite video editor or ffmpeg.The steps for downloading the video file is the same except that you need to discard the range parameter from the Request URL of the video file. This is to ensure that the complete video file is downloaded.
The steps for downloading the audio file is also the same except for step 3. The audio and video file have the same name and the only way of distinguishing between them is checking their file type. So, in order to download the audio file you need to select a videoplayback file which has a file type of audio/mp4 and then continue with the remaining steps.
Lastly, due to the obvious complexity of this method, I would like to share an alternative less complex method of downloading videos not only from youtube but from any video streaming website. The method involves a command-line program called youtube-dl available from youtube-dl.org. To download a youtube video using youtube-dl, you need to run the program followed by the URL of the youtube video. Example:
> youtube-dl http://www.youtube.com/watch?v=[youtube_video_id]
where youtube_video_id is the video id of the youtube video.
