A word of warning, after prolonged use the case of T3 (BFY51) does get quite hot. The peak current through the speaker and the transistor is quite high - around 800mA, so take care when testing.
The circuit works with a 9v PP3 battery which will gives many weeks of operation. The peak current is quite high (few hundred mA's) but this occurs on each beat. The normal operating current is around 12mA. Diode D3 prevents the circuit from being damaged if the battery is momentarily connected the wrong way.
Download PCB and circuit details