How to add audio in desktop capture in chrome extension

I have added audio to chrome extension on screen cast of desktop. But audio is not in good quality. As, I record screen of the tab. I got good quality video and audio. How can I get good quality audio in desktop recording. Here is what I am using

 chrome.desktopCapture.chooseDesktopMedia(['screen', 'window', 'audio'], function (streamId) {
        if (streamId) {
              var obj = {
               audio: {
                mandatory: {
                        chromeMediaSource: 'desktop',
                        chromeMediaSourceId: streamId
                    }
               },
               video: {
                optional: [],
                   mandatory: {
                      chromeMediaSource: 'desktop',
                      chromeMediaSourceId: streamId,
                      maxWidth: 2560,
                      maxHeight: 1440
                   }
               }
            };

            countdownRun(function () {
                window.navigator.mediaDevices.getUserMedia(obj).then(function(stream) {
    $streamVideo = stream;
    $timeStart = Date.now();
    var audio = stream.getAudioTracks()[0];
    var video = stream.getVideoTracks()[0];
    alert(JSON.stringify(audio))
    alert(JSON.stringify(video))
    if (micSound) {
        window.navigator.webkitGetUserMedia({audio: true}, function (s) {
            $streamAudio = s;
            audio = s.getAudioTracks()[0];
            captureUseNacl(audio, video);
        }, function (e) {
            chrome.tabs.create({url: 'mic.html'});
        })
    } else {
        captureUseNacl(audio, video);

        (function () {
            var v = document.createElement('video');
            document.body.appendChild(v);
            v.setAttribute('autoplay', '');
            v.addEventListener('canplay', function () {
                console.log('play video');
            }, false);
            v.src = window.URL.createObjectURL(stream);
            $streamElement = v;
        })()
    }
}).catch(function(err) {
alert(err)
    alert(JSON.stringify(err));
});;
            })
        }
    });