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EricCartman On DASH videos (Adaptive Video Streaming)
YouTube now uses adaptive video streaming for 480p and 1080p videos (Oct 2013). What this means is that the video and audio streams are now separate.
They are indicated as M4V and M4A. They have to be downloaded separately and mux'ed before playing. If using ffmpeg:
ffmpeg -i vid.m4v -i vid.m4a -vcodec copy -acodec copy vid.mp4
The filename is always videoplayback. Before saving, select and copy the page title first. Then right-click on the video link, save-as and paste the title. Set the extension to either m4v or m4a to distinguish between video and audio.