connect-instrument.js

const context = new AudioContext();

if (context.state === 'suspended') {
  await context.resume();
}

const stream = await navigator.mediaDevices
  .getUserMedia({
    audio: {
      echoCancellation: false,
      autoGainControl: false,
      noiseSuppression: false,
      latency: 0
    }
  });
const lineInSource = context.createMediaStreamSource(stream);

lineInSource.connect(context.destination);