RecordRTC: How do you capture the audio mediastream in Chrome?

Question

So I've been trying to use RecordRTC to record the audio coming from a tab. RecordRTC needs to be passed Mediastream as an argument which requires the stream from the page to be captured. Linked some code down here which tries to capture that stream but can't get it to work. Also will be linking my github repository: https://github.com/laotsunami/soundbyte

Help please!

var constraints = {audio: true, video: false};
var bg = chrome.extension.getBackgroundPage();
bg.console.log("started");


document.addEventListener('DOMContentLoaded', function() {


function successCallback(stream) {
  bg.console.log('entered success callback')
  window.stream = stream; // stream available to console
  if (window.URL) {
    audio.src = window.URL.createObjectURL(stream);
  } else {
    audio.src = stream;
  }
}

function errorCallback(error){
  bg.console.log('navigator.getUserMedia error: ', error);
}

navigator.getUserMedia(constraints, successCallback, errorCallback);

  // bg.console.log(mediaStream);

  var startButton = document.getElementById('startButton');
  startButton.addEventListener('click', function() {
    bg.console.log('Clicked Start Button');

    
    var recordRTC = RecordRTC(mediaStream); 
    bg.console.log('Clicked Start Button');
    recordRTC.startRecording(function() {
      bg.console.log('recording started');
    }); 
    bg.console.log("didn't crash");
  });

  var stopButton = document.getElementById('stopButton');
  stopButton.addEventListener('click', function() {
    bg.console.log('Clicked Stop Button');

    recordRTC.stopRecording(function(audioURL) { 
          bg.console.log('stopped recording');
          mediaElement.src = audioURL;
          bg.console.log(audioURL); 
      });
  });

});