audio.addEventListener('playing', playingEventHandler);

function playingEventHandler() {
    audio.removeEventListener("playing", playingEventHandler);
    setTimeout(function() {
        fadeOut(audio, 3000);
    }, 10 * 1000);
}

function fadeOut(audio, fadeDuration, initialVolume) {
  var stepSize = (initialVolume || audio.volume) / (fadeDuration / 50);
  var volume = Math.max(0, audio.volume - stepSize);

   audio.volume = volume;

   if(audio.volume > 0) {
        // Keep up going until 11
        setTimeout(function() {
            fadeOut(audio, fadeDuration, initialVolume)
        }, 50);
    }
}