#define BAUD_RATE    38400
#define SPEAKER      9
#define SCALE_FACTOR 4

void setup()
{
   Serial.begin( BAUD_RATE );
   pinMode( SPEAKER, OUTPUT );
}

void loop()
{
   if( Serial.available() > 1 )
      analogWrite( SPEAKER, ( Serial.read() / SCALE_FACTOR ) );
}