Playing music with an Arduino
Thanks to Mikael Patel for suggesting that link (your Google-Fu must be strong!).

I am pasting the code below so that this answer survives any possible future deletion of the read-only part of the Arduino Forum.

//ONG LIT YIT 2013-07-20
//This following code plays happy birthday melody on Arduino
//Put Piezo Buzzer on GDN and 9 (Positive and negative are reversible)
//this project requires a Piezo Buzzer and

// an Arduino board and
//jumper wires to connect Buzzer's (+) to ~9 and (-) to GND (any GND)

const int speakerPin = 9;
const int LENGTH = 28; // the number of notes
const char notes[LENGTH + 1] = "GGAGcB GGAGdc GGxecBA yyecdc";
const int beats[LENGTH] = { 2, 2, 8, 8, 8, 16, 1, 2, 2, 8, 8, 8, 16, 1, 2, 2, 8, 8, 8, 8, 16, 1, 2, 2, 8, 8, 8, 16 };
const int tempo = 150;

void playTone(int tone, int duration) 
  {
  for (long i = 0; i &lt; duration * 1000L; i += tone * 2) 
    {
    digitalWrite(speakerPin, HIGH);
    delayMicroseconds(tone);
    digitalWrite(speakerPin, LOW);
    delayMicroseconds(tone);
    }
  } // end of playTone

void playNote(const char note, const int duration) 
  {
  char names[] = {'C', 'D', 'E', 'F', 'G', 'A', 'B',
                  'c', 'd', 'e', 'f', 'g', 'a', 'b',
                  'x', 'y'
                 };
  const int tones[] = { 1915, 1700, 1519, 1432, 1275, 1136, 1014,
                  956,  834,  765,  593,  468,  346,  224,
                  655 , 715
                };
  const int SPEE = 5;

  // play the tone corresponding to the note name
  for (int i = 0; i &lt; 17; i++) 
    {
    if (names[i] == note) {
      int newduration = duration / SPEE;
      playTone(tones[i], newduration);
    }
  }
} // end of playNote

void setup() 
  {
  pinMode(speakerPin, OUTPUT);
  } // end of setup

void loop() 
  {
  for (int i = 0; i &lt; LENGTH; i++) 
    {
    if (notes[i] == ' ') 
      {
      delay(beats[i] * tempo); // rest
      } 
    else 
    {
      playNote(notes[i], beats[i] * tempo);
    }

    // pause between notes
    delay(tempo);
  }
} // end of loop


I also cleaned up the code a bit.