Next, try to see if you have just a single MP4 file that's giving you problems or more than one. Make sure to check several MP4 files that you're certain played correctly in the past and check them all in a single media player. If there is just one file that's giving you a problem and the others work just fine, it's most likely Cause #3. If all MP4 files suddenly refuse to play sound or function at all, it is most likely Cause #2.