MediaRecorder: свойство stream

if (navigator.getUserMedia) {
  console.log("getUserMedia supported.");
  navigator.getUserMedia(
    // constraints - only audio needed for this app
    {
      audio: true,
    },

    // Success callback
    (stream) => {
      const mediaRecorder = new MediaRecorder(stream);

      const myStream = mediaRecorder.stream;
      console.log(myStream);
    },
  );
}