29 Apr MP4 with No Audio

Today’s (29 Apr 2016) MP4

A History of the United States in 11 Minutes has no audio track. Other new MP4 files do. Ken

@branko Today I received two Spanish folders with mp4 videos under Khan Academy.
I tried to play them and found that they have no video and audio. Just a blank clip is playing.

EDIT- I can hear audio now after increase the volume between 90-100.