Regarding the above, not sure what happen since previously I could play any files I download from youtube, be it avi or mp4.
Now I cannot play any mp4 file downloaded from youtube but no issue to play any other file format (avi, mov, flv using VLC. Some other mp4 files that was downloaded from different source other than youtube are able to play.
vlc -vv *mp4 file of your choice*
in a terminal and look for errors? this might help too: askubuntu.com/questions/166311/…