<!DOCTYPE html>
<html>
<head>
    <style>
        div {
            width: 400px;
            height: 400px;
            background-color: red;
        }
        body {
            margin: 0;
        }
    </style>
    <script>
        var audio;

        function playFirstTrack()
        {
            audio = document.createElement("audio");
            audio.src = "video-with-audio.mp4";
            audio.id = "first";
            audio.onplaying = audioPlayingHandler("first");
            audio.onended = playSecondTrack;
            audio.play();
        }

        function playSecondTrack()
        {
            audio = document.createElement("audio");
            audio.src = "large-video-with-audio.mp4";
            audio.id = "second";
            audio.onplaying = audioPlayingHandler("second");
            audio.play();
        }

        function seekToEnd()
        {
            audio.currentTime = audio.duration - 0.5;
        }

        function audioPlayingHandler(id)
        {
            return function() {
                try {
                    window.webkit.messageHandlers.testHandler.postMessage("playing-" + id);
                } catch(e) {}
            }
        }
    </script>
</head>
<body>
    <div onmousedown=playFirstTrack()>Click me to start playing!</div>
</body>
</html>