demo.css

* {
  box-sizing: border-box
}
html, body {
  margin: 0;
  font-family: Arial, sans-serif;
  color: #555;
  font-size: 16px;
}

a {
  color: #1976D2;
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

.header {
  padding: 8px;
}

.header h1 {
  color: #333;
  margin: 10px 0;
  padding: 0 0 10px;
  border-bottom: 1px solid #ccc;
}

.header p {
  margin: 15px 0;
  padding: 15px 0;
  border-bottom: 1px solid #dedede;
}

.container {
  width: 100%;
  max-width: 1000px;
  padding: 8px;
  margin: auto;
}

.audio__box {
  width: 100%;
  padding: 5px;
  margin: 25px auto;
  background-color: #000;
}

.audio__box audio {
  width: 100%;
}

.audio__box canvas {
  width: 100%;
  height: calc(100% / 4);
  background-color: #002D3C;
}

.control {
  display: flex;
  margin-bottom: 10px;
}

.control label {
  width: 30%;
  padding: 4px 6px;
}

.control select {
  flex: 1;
  height: 26px;
}