MP4 does not play in Unity

Some of the MP4 files does not play in the unity. Could anybody know what could the problem? Is this something with the codec used to convert mp4?

Yes, Unity does not support mp4 files. I suspect the files that do work are simply mp3 audio wrapped up inside an mp4 container.

I know this is really old, but just want to update anyone reading this. Unity does now support mp4 videos. Unity - Manual: Movie Textures