How can I add predefined length to audio recorded from MediaRecorder in Chrome?

function exportAudio(blob) {
  const aud = document.getElementById("aud");
  aud.src = URL.createObjectURL(blob);
  aud.addEventListener("loadedmetadata", () => {
    // It should have been already available here
    console.log("duration:", aud.duration);
    // Handle chrome's bug
    if (aud.duration === Infinity) {
      // Set it to bigger than the actual duration
      aud.currentTime = 1e101;
      aud.addEventListener("timeupdate", () => {
        console.log("after workaround:", aud.duration);
        aud.currentTime = 0;
      }, { once: true });
    }
  });
}

// We need user-activation
document.getElementById("button").onclick = async ({ target }) => {
  target.remove();
  const resp = await fetch("https://upload.wikimedia.org/wikipedia/commons/4/4b/011229beowulf_grendel.ogg");
  const audioData = await resp.arrayBuffer();
  const ctx = new AudioContext();
  const audioBuf = await ctx.decodeAudioData(audioData);
  const source = ctx.createBufferSource();
  source.buffer = audioBuf;
  const dest = ctx.createMediaStreamDestination();
  source.connect(dest);

  const recorder = new MediaRecorder(dest.stream);
  const chunks = [];
  recorder.ondataavailable = ({data}) => chunks.push(data);
  recorder.onstop = () => exportAudio(new Blob(chunks));
  source.start(0);
  recorder.start();
  console.log("Recording...");
  // Record only 5 seconds
  setTimeout(function() {
    recorder.stop();
  }, 5000);

}

<button id="button">start</button>
<audio id="aud" controls></audio>