The amplifier operates in a way similar to pulse-width modulation. The 555 modulates the Audio input with a frequency higher than the speaker response capability so the modulating frequency is dropped (the coil in the speaker acts as a HF filter) and the input signal is being played by the speaker
Note1: Adding a 100n cap between pin 1 and 2 will dramatically increase the volume, distortion and heat up the chip very fast. Also (from original circuit) adding a capacitor across the speaker terminals will increase the volume and distort the output but without heating the chip.
Note2: To add a volume control, replace the 100K connected between the Transistor base and ground with 100K Pot.
Note3: The transistor used in this circuit is BC547, the IC is NE555 and the actual DC supply is 12V.
Using iCircuit App for iPhone