The MediaStream represents synchronized streams of media. If there is no audio tracks, it returns an empty array and it will check video stream, if webcam connected, stream.getVideoTracks() returns an array of one MediaStreamTrack representing the stream from the webcam.
function gotStream(stream) {
window.AudioContext = window.AudioContext || window.webkitAudioContext;
var audioContext = new AudioContext();
// Create an AudioNode from the stream
var mediaStreamSource = audioContext.createMediaStreamSource(stream);
// Connect it to destination to hear yourself
// or any other node for processing!
mediaStreamSource.connect(audioContext.destination);
}
navigator.getUserMedia({audio:true}, gotStream);