Compatible with iOS 9
For iOS 6,7,8, try Volume Mixer
Volume Mixer 2 allows you to control the volume of each media channel & output separately, instead of being able to change it only when that channel is active (Ringer, Audio/Video for music, video and games, Phone Call, Bluetooth headset & headphones). You can display all of the controls in the mixer at the same time, or choose to hide the ones you do not need.
Several skins are included, in addition to stock iOS look (which you can fully customize by changing colors of text, sliders and thumb grabbers).
You can configure it in Settings/Volume Mixer 2, and open it using any Activator action. Default actions for opening are long press on Volume Up button or tapping on the volume HUD – you can change it in Settings. Close the mixer by tapping anywhere outside it.
Supported on all iOS 9 devices. Free if you have purchased old Volume Mixer previously.