The AudioMixer is a very self-contained assetAny media or data that can be used in your game or Project. An asset may come from a file created outside of Unity, such as a 3D model, an audio file or an image. You can also create some asset types in Unity, such as an Animator Controller, an Audio Mixer or a Render Texture. More info
See in Glossary with a streamlined API.
A couple of music-related demos can be found in the audio demos repository.
Also the native audio plugin repository provides the official SDK for developing custom extensions as well as many examples of such involving complex routing and custom processing/generation of audio in the mixer.