Watching a video like this is a great way to not only learn how to build a circuit, but also see how the right tools make testing easier.
Youtube user w2aew explains the parts of a circuit used to attenuate an audio signal to a constant amplitude. For example, audio leveling is used in music production to minimize the range of volume of the track. In another case, audio leveling can level out the volume two tracks in your playlist so you don’t have to manually adjust the volume of your player. An overview of the more complicated concept of Dynamic Range Compression can be read at your favorite information source, Wikipedia.
I think it’s worth noting the use of the function generator and four-channel oscilloscope in the video. It’s really nice to be able to visualize the functionality different parts of your circuit, which will make finding problems that much easier.