This is the circuit diagram of a simple three transistor audio amplifier that can deliver around 100mW power to a 25 Ohm speaker. The diodes D1 and D2 provides a constant bias voltage for the transistors Q1 and Q2.The transistor Q1 works as a preamplifier. Transistors Q2 and Q3 drives the speaker. The type no of the transistor is not very crucial here. You can use any NPN transistor for Q1, Q3 and any PNP transistor for Q2.Any way the minimum collector current capacity of the transistors must not be less than 100mA.The circuit will work well with an 8 ohm speaker too, but the volume will be a little less.
The circuit can be assembled on a Vero board. A 9V PP3 battery can be used to power the circuit. Loud speaker K1 can be a 25 Ohms one. Capacitor C1 and C2 must be rated at least 15V.