C1815 transistor can be used in audio amplification stages, small audio amplifier (if used to drive small speaker), preamplifier and also in preamplifier stages. 

C1815 is a widely used transistor, it is used in commercial and educational projects. It is designed for audio frequency amplification and high frequency OSC. The collector-base voltage of the transistor is 50V therefore it can be easily used in circuits using below 50V DC. The collector current of the transistor is 150mA therefore it can drive any load below the 150mA limit. The collector power dissipation and DC current gain values of the transistor is quite good due to which it is ideal to use in audio amplification and electronic signal amplification purposes. Moreover it can also be used as a switch to drive loads under 150mA.