​

function setup() {

  song = loadSound('assets/test.mp3');

  createCanvas(720, 200);

  background(255, 0, 0);

}

​

function mousePressed() {

  if (song.isPlaying()) {

    // .isPlaying() returns a boolean

    song.stop();

    background(255, 0, 0);

  } else {

    song.play();

    background(0, 255, 0);

  }

}

​