The transistor is a switch that can be operated by applying a small voltage to one of the legs, causing another leg to "open" or "close". Which leg does what is dependant on the type of transistor, which is why I always double check the datasheet. Transistors can be influenced by the ambient temprature. [^touch], and therefore are usually placed in pairs,
cancelling each other out.
[^touch]: When circuit bending, transistors are great to touch, as the heat of your finger could alter the sound.