IDArnhem: This code must be loaded in the Arduino and it runs the buzzer.

buzzer.ino

/**
 * This little program runs in the Arduino, one that is plugged
 * to a computer with a USB cable. This cable will be our "serial"
 * connection. In the computer we will run the P5 sketch that will
 * tell the Arduino through "serial" when to do its thing.
 * 
 * 1. Load this program into your Arduino.
 * 2. Load the Processing sketch, change the "serial port" in the P5 code to point to the correct Arduino.
 * 3. Run the processing sketch.
 * 4. Buzzzzzz
 */
int DATA_RATE = 9600;
int buzzer = 8;

byte msg = 0;

void setup() {
  Serial.begin(DATA_RATE);
  delay(30); // wait to make sure serial is open and fine
  pinMode(buzzer, OUTPUT);
}

void buzz() {
  tone(buzzer, 1500); // play 400 Hz tone for 500 ms
  delay(300);
  noTone(buzzer);
}

void loop() {
  //  check if data has been sent from the computer to the arduino
  if (Serial.available()) {
    // if there's data waiting, read the most recent byte
    msg = Serial.read();
    // if we get a capital B from the computer, we buzz the buzzer
    if(msg == 'B') {
      buzz();
    }
  }
  delay(100);
}