int buzzer_pin = 4; 
int volume;
void setup() { 
   Serial.begin(9600);
  pinMode(buzzer_pin, OUTPUT); 
    Serial.write("Enter volume:");
} 
void loop() { 
if (Serial.available() == 0)   
    {  
    volume = Serial.parseInt();
    }
  analogWrite(buzzer_pin, volume); 
}