<!DOCTYPE html>
<html>
<head>
    <script>
    function go() {
        var audio = document.getElementsByTagName('audio')[0];
        audio.play().then(playing).catch(notPlaying);
    }

    function playing() {
        try {
            window.webkit.messageHandlers.testHandler.postMessage('playing');
        } catch(e) {
            window.location = 'callback:playing';
        }
    }

    function notPlaying() {
        try {
            window.webkit.messageHandlers.testHandler.postMessage('not playing');
        } catch(e) { }
    }
    </script>
</head>
<body onload="go()">
    <audio src="video-with-audio.mp4"></video>
</body>
</html>