It isnt a sound amplifier !? Its a switching circuit? (so I belive)
If U'd collector to 12V and emitter to GND - then it would be very important to control the current, to avoid short circuit!
In your case, you have a motor involved on the collector side. This shall be switched on or off?
OK. If u put a low voltage (<.5V) on base - the transistor breaks any connection C-E -> no current flow
When u saturate the transistor by driving enough current into its base, Vce drops to belov 1V and the transistor if fully open for current to flow. The motor (resistor) in the collectors path decides how much current that will flow. You said that your fan is marked 150mA
A chart in the datasheet indicates that you need more than microamps base current to achieve saturation.
Try increasing the base resitor and observe that the Vce rises -> and the transitor gets hot.
In a switching circuit you should reduse base resistance until transitor saturates.
You will even hear that the fan/motor slows down if base resistor is to big.