Adjusts the volume of the most relevant stream. For example, if a call is active, it will have the highest priority regardless of if the in-call screen is showing. Another example, if music is playing in the background and a call is not active, the music stream will be adjusted.

This method should only be used by applications that replace the platform-wide management of audio settings or the main telephony application.

This method has no effect if the device implements a fixed volume policy as indicated by AudioManager.IsVolumeFixed.

[Android Documentation]