setRelativeVolume

Use the setRelativeVolume command to increase or decrease group volume and set the group's mute state. The group is automatically unmuted unless otherwise specified.

Your app can use setRelativeVolume instead of setVolume when the user intent is to increase or decrease the group volume, but not to set the final volume to a particular value. For example, your app should use setRelativeVolume when the user presses hard volume plus/minus buttons on a mobile device.

The group will automatically limit the final volume set within the valid range, so your app does not need to worry about that.

See setVolume for more details on how group volume works.