A - First, check to make sure if the video description says "No Audio". Some movies have no audio track. If the movie is supposed to have sound, but you can't hear it, try the following:
1) Download QuickTime for MP4 and MOV movie files.
2) Download VLC for MP4 and MOV movie files.
3) Download K-Lite Codec Pack for older PC playback compatibility.
4) Download Perian for Mac playback compatibility.