const int SOUND_SENSOR_PIN = 9;
int lastState = HIGH;
void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(SOUND_SENSOR_PIN, INPUT);  
}

void loop() {
  // put your main code here, to run repeatedly:
  int currentState = digitalRead(SOUND_SENSOR_PIN);

  if (lastState == HIGH && currentState == LOW)
    Serial.println("The sound has been detected");
  else if (lastState == LOW && currentState == HIGH)
    Serial.println("The sound has disappeared");

}