Edit in JSFiddle

let pc1 = new RTCPeerConnection(), pc2 = new RTCPeerConnection(), stream, videoTrack, videoSender;

(async () => {
  try {
    stream = await navigator.mediaDevices.getUserMedia({video: true, audio: true});
    videoTrack = stream.getVideoTracks()[0];
    pc1.addTrack(stream.getAudioTracks()[0], stream);
  } catch (e) {
    console.log(e);  
  }
})();

checkbox.onclick = () => {
  if (checkbox.checked) {
    videoSender = pc1.addTrack(videoTrack, stream);
  } else {
    pc1.removeTrack(videoSender);
  }
}

pc2.ontrack = e => {
  video.srcObject = e.streams[0];
  e.track.onended = e => video.srcObject = video.srcObject; // Chrome/Firefox bug
}

pc1.onicecandidate = e => pc2.addIceCandidate(e.candidate);
pc2.onicecandidate = e => pc1.addIceCandidate(e.candidate);
pc1.onnegotiationneeded = async e => {
  try {
    await pc1.setLocalDescription(await pc1.createOffer());
    await pc2.setRemoteDescription(pc1.localDescription);
    await pc2.setLocalDescription(await pc2.createAnswer());
    await pc1.setRemoteDescription(pc2.localDescription);
  } catch (e) {
    console.log(e);  
  }
}
<label><input type="checkbox" id="checkbox">Add video to audio-call (turn this on and off) </label>
<button onclick="stream.getTracks().forEach(t => t.stop())">Stop Example</button><br>
<video id="video" width="320" height="240" autoplay controls></video>
<script src="https://webrtc.github.io/adapter/adapter-latest.js"></script>