Arduino Cookbook - Cdn.oreilly.com

Figure 1-9. Connections for a speaker with the LDR circuit
You will need to increase the on/off rate on the pin to a frequency in the audio spectrum.
This is achieved, as shown in the following code, by decreasing the min and max
durations:
const int outputPin = 9; // Speaker connected to digital pin 9
const int sensorPin = 0; // connect sensor to analog input 0
const int minDuration = 1; // 1ms on, 1ms off (500 Hz)
const int maxDuration = 10; // 10ms on, 10ms off (50 hz)
void setup()
{
pinMode(outputPin, OUTPUT); // enable output on the led pin
}
void loop()
{
int sensorReading = analogRead(sensorPin); // read the analog input
int rate = map(sensorReading, 200,800,minDuration, maxDuration);
rate = constrain(rate, minDuration,maxDuration); // constrain the value
digitalWrite(outputPin, HIGH); // set the LED on
delay(rate); // wait duration dependent on light level
digitalWrite(outputPin, LOW); // set the LED off
delay(rate);
}
20 | Chapter 1: Getting Started