Extract audio from a video blob with vanilla JavaScript

On a web page, after recording screen video and audio using navigator.mediaDevices.getDisplayMedia and a MediaRecorder, I end up with a video blob. How can I extract using only vanilla JavaScript on the client the audio from that blob and for example have it downloaded to the user? I'm simply looking for this function:

async function extractAudio(videoBlob) {
  // ...

  return audioBlob;
}