Video embed field doesn't honor the autoplay settings
Use the following code to change autoplay to 1 in your template or Javascript file. It works for me.

jQuery('.video-embed-field-launch-modal').attr('data-video-embed-field-modal', function(i, oldSrc) {
  var isAutoPlay = oldSrc.match(/autoplay=([^&amp;]*)/);            
  if (isAutoPlay &amp;&amp; isAutoPlay[1] == 0) {
    return oldSrc.replace('autoplay=0', 'autoplay=1');
  }
});