How to get audio input and audio output as one stream?

You may do it by the following code, (Using MediaStream Recording API approach)

var audioContext = new AudioContext();
var mediaStreamDest = new MediaStreamAudioDestinationNode(audioContext);
document.querySelectorAll("audio").forEach((e) => {
  var mediaElementSource = new MediaElementAudioSourceNode(audioContext, { mediaElement: e });
  mediaElementSource.connect(mediaStreamDest);
});
console.log(mediaStreamDest.stream.getAudioTracks()[0]);

The above line, given in the console log will get the first audio track from your MediaStream. 

stream.addTrack(mediaStreamDest.stream.getAudioTracks()[0])

After that, it will mix up audio streams into one stream.