Sometimes, this needed to control sound in developer app. For example, when user listening music in background and app turns on sound notification, they both mixed and notification not listened. With another extension I can mute music stream, and turn on the sound notification.
Also, needed same ability in text-to-speech component. I'm making app for blind persons.
you might want to try the settings extensions, which offers methods to set different volumes: ring, music and alarm
https://puravidaapps.com/settings.php
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
Thank you for your response. Yes, I meant this extension when wrote. But, for me problem in Sound component, which doesn't allow to change stream. If I will mute music stream, this mutes my sound notifications also. Changing of stream in Sound component to ring may solve this problem. I wrote an app for blind persons with voice notifications, and obtain a feedback that if they listening music during my app working, voice notifications become unapparent.