I used to mess around with audio electronics some time ago. The transistor is the real workhorse for a lot of audio components. Take the amplifier mentioned in the article. The transistor does most of the work for you.
So what that means is that the rest of the circuit design is fairly simple. You have a few resistors here and there, but basically the circuit diagram is mostly a bunch of these 2N3904 NPN transistor components strung along together to create the increased amplifier effect.
Each transistor creates a certain amplification effect. You can then take the output of one transistor and feed it as the input to the next one, and so on, eventually multiplying the amplification as you need it. At a ratio of 100 times input to output, you can see how you can build a really strong amplifier in short order, with just a handful of components.