The AudioMixer is a very self-contained asset 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.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.