WebKit's createMediaStreamSource has a bug which requires it to be called very soon after the stream is created. If called after a delay it appears to produce a silent webaudio node. 
navigator.mediaDevices.getUserMedia({ audio: true }).then((stream) => {
  cleanupJobs.push(() => stream.getTracks().forEach(track => track.stop()));

  if (useDelay) {
    // Bug occurs
    return delay(500).then(() => stream);
  }

  // Bug does not occur
  return stream;
}).then((stream) => {
  const source = audioCtx.createMediaStreamSource(stream);

  // ... Visualize audio data from source

};