Switching between circuits, transistor-based SPDT?
The circuit you are (will be) building is this one which is a pretty standard one transistor amplifier and they go through its design step by step.


The makers of the ceramic buzzer give a number of driver circuits, the simplest is this one which can be driven by an external oscillator (e.g 555 astable, cmos multivibrator etc.) or a pin from a microcontroller.



To combine the two circuits just use a simple SPDT switch. Sometimes using transistors as switches over complicates the design. 

You only need a transistor switch if you are going to change over the function by some remote device.