MediaStream에서 사운드 스트림을 추출하여 좌우로 분리된 스트림을 만듭니다.

function getAudioStream(stream, channel){
  const audioCtx = new AudioContext();
  const dest = audioCtx.createMediaStreamDestination();
  const merger = audioCtx.createChannelMerger(2);
  stream.getAudioTracks().forEach((track, index) => {
    const stream = new MediaStream([track]);
    const mutedAudio = new Audio();
    mutedAudio.muted = true;
    mutedAudio.srcObject = stream;
    mutedAudio.play();
    const source = audioCtx.createMediaStreamSource(stream);
    source.connect(merger, 0, channel);
  })
  merger.connect(dest);
  return dest.stream;
}

참고 자료