<a href="http://casperelectronics.com/wp-content/uploads/Interfacing/Triggering.jpg"><img src="http://casperelectronics.com/wp-content/uploads/Interfacing/Triggering.jpg" alt="" title="Triggering" width="600" height="483" class="alignnone size-full wp-image-757" /></a>

The drawing above shows one of the many ways you can utilize transistors in circuit bending. 
The transistor in this drawing is there to automatically trigger the circuit to make sound. The circuit shown is a generic sound board from a generic kids toy.<p>
Let's go over the basics. The black circle is the brain where the real work of the circuit is done and the sound is recorded or generated. To the right of that is a pitch resistor above is a transistor which acts as an amplifier for the speaker. next to that are two button contacts. when these are connected it triggers whatever the circuit is meant to do.

I've got a situation where I want the circuit to continuously make sound. In some cases you can simply wire a jumper across the button contacts and it will keep triggering sounds. In MANY cases the brain wants to "see" a new button press in order to trigger a sound. Want I want in most cases is for the sound to trigger immediately after the sound has finished, and that's what the transistor does. 
One important factor in this circuit is that everything is tied to the positive power, meaning the button contacts are connected to the trigger in(on the brain) and power, the pitch is pitch in and power, the speaker is amp(transistor) out and power. This is important to note when using transistors because the transistors will basically allow you to control the current which goes into or out of these points (trigger, pitch, audio). Knowing what the brain wants to "see" will tell you what kind of transistor configuration to use. 
In this case the brain wants to see positive voltage at the trigger point. The emitter of the transistor is connected to the trigger pin and the collector is connected to the supply voltage (4.5V). When  the BASE pin of the transistor is connected to ground or negative voltage, current will flow between the emitter and collector, which will trigger the circuit to make sound. <br>The BASE pin in this circuit is connected to the audio out and is then pulled low (to ground) through a 100k resistor. That means when there is no signal at the base it will be pulled low (current will flow between collector and emitter). When there is a signal (ie. sound is being generated) there will be NO flow between collector and emitter. So as soon as it stops making sound, the lack of sound activates the transistor which triggers another sound. 