This is a simple 3 component device that amplifies the sound Arduino can produce roughly by a factor of 10!
Basically the way it works is as follows:
A pin on the Arduino is oscillating (turning on and off) thus creating a sound with a particular frequency. The signal from this pin is connected to the base of the BD135 transistor which is powered by a 9V battery. The amplification is in the order of 10- 20 (500mA X 9V vs 50mA X 5V) but to spare your battery and obtain an enjoyable volume the circuit works better with a 10K variable resistor on the base of the BD 135.
•10K Variable Resistor
•BD 135 Transistor
Thats about it, your turn to build it!!
p.s: free download of schematic and pdf handout available here: http://robotix.com.au/tutorials2.html