Transistor substitution guidelines? (and the BC147A)
IN low frequency (audio) circuits the most important characteristics generally are:-


Polarity: (NPN or PNP)
Collector-Emitter breakdown voltage: should be at least 20% higher than the supply voltage.
Current gain (hFE): should be similar.
Maximum current: Should be much higher than the operating current. 
Maximum power: This is usually similar for the same package size.


The BC147B is NPN, 45 V breakdown, hFE 200 minimum, and max current 200 mA.

The popular 2N3904 is 40 V breakdown, hFE 100-300 at 10 mA, max current 200 mA. These specs are similar enough that it will probably work fine, provided the supply voltage is not over 33 V.