I've taken some MP4 videos on my Nokia Lumia 720 and downloaded them to my PC (WinXP). They won't play at all in Windows Media Player. I converted them to AVI files using two different converter programs, and they do play but with audio only.
How can I play them properly on my PC?
Your computer is missing a "codec" to decode the video data into something viewable. Windows 7 and 8 come with a lot of these codecs out-of-the-box, so apart from protecting yourself from security threats such as worms and malware, that may be another reason to upgrade (you should seriously consider this option).
As a more immediate solution, the open-source video player VLC comes bundled with a lot of codecs, so it can probably play those videos.
According to Microsoft support website, Windows Media Player still won't support MP4 files. Though we can make WMP play MP4 video by install some codec, sometime we still encounter can't play MP4 in Windows Media Player issue. MP4 is a container format which may contain multiple video and audio streams. And the video and audio stream can be encoded any of several ways. If the MP4 video or audio codec is not WMP installed codec supported one, Windows Media Player won't play MP4 video of this kind.
In order to solve "Windows Media Player won't play MP4 files even when the appropriate codec is installed" and to successfully play any MP4 video in Windows Media Player, we had better convert MP4 to Windows Media Player more supported video format like WMV, AVI.
Resources: http://www.idealshare.net/video-converter/windows-media-player-mp4.html
