video-sync.html

<video id="myvideo" controls muted loop>
    <source src="sample_video_no_sound.mp4" type="video/mp4"/>
    <audio id="myaudio">
        <source src="sound.mp3" type="audio/mpeg"/>
    </audio>
</video>

<script>
    var myvideo = document.getElementById("myvideo");
    var myaudio = document.getElementById("myaudio");

    var change_time_state = true;

    myvideo.onplay = function(){
        myaudio.play();
        if(change_time_state){
            myaudio.currentTime = myvideo.currentTime;
            change_time_state = false;
        }
    }

    myvideo.onpause = function(){
        myaudio.pause();
        change_time_state = true;
    }
</script>