var videos = document.querySelectorAll('.myvideo');
       
       function setRandomPlaybackRate() {
           var minRate = 0.0;
           var maxRate = 0.4;
           var randomRate = Math.random() * (maxRate - minRate) + minRate;
            
           videos.forEach(function (video) {
               video.playbackRate = randomRate;
           });
       }
       
       
       videos.forEach(function (video) {
           video.addEventListener('play', function () {
               setInterval(setRandomPlaybackRate, 4000);    
           });
       });