Critical Code

var mic;

function setup() {
  var myCanvas = createCanvas(800, 200);
  myCanvas.parent('myContainer');
  mic = new p5.AudioIn();
  mic.start();
}

function draw() {
  background(0);
  var vol = mic.getLevel();
  fill(210, 50, 103);
  noStroke();
  var h = map(vol, 0, 1, 50, 800);
  ellipse(width/2, height/2, h, h);
}