a.html

<html>
<head>
<script>
  const handleSuccess = function(stream) {
    const context = new AudioContext();
    const source = context.createMediaStreamSource(stream);
    const processor = context.createScriptProcessor(1024, 1, 1);
    source.connect(processor);
    processor.connect(context.destination);
    processor.onaudioprocess = function(e) {
      // Do something with the data, e.g. convert it to WAV
      // console.log(e.inputBuffer);
      var x = e.inputBuffer.getChannelData(0);
      var j = 0
      for (let i = 1; i < x.length; i++) {
        if (x[i-1] * x[i] < 0) j++;
      }
      document.getElementById('out').innerHTML = String(j/(x.length-1)*e.inputBuffer.sampleRate/2);
    };
  };
  navigator.mediaDevices.getUserMedia({ audio: true, video: false }).then(handleSuccess);
</script>
</head>
<body>
<div id="out"></div>
</body>
</html>