I am having this problem, but just with one, single, video.  I take the MP4, which can be watched in other programs and watched and edited in Microsoft Movie Maker.  I then open it up in Movie Maker and add an additional audio track.  It plays just fine in Movie Maker.  When I export it (tried several formats, mp4 and windows media player format) and the exported file cannot be opened by media player.  Media player states that it "encountered a problem" when it tried to play the file.  Right clicking and checking properties of the files shows none of the normal info for a video file (like duration, resolution, etc) as if it wasn't even recognized as a video file.   Everything going in works fine, just what comes out doesn't work.  Tried it on two different computers, one with a fresh essentials install.  Still nothing.  Did DXdiag, nothing came up.  Display drivers fully up to date, even did a fresh install of codecs.  But I can take other mp4 files in movie maker and audio files (same formats) and export them from movie maker and they're fine.  Any ideas?