LG Audio - I cannot control volume.

I cannot control volume.
-Target model : Smart audio(Music Flow)_NP8*40/NP8350, Sound bar_LAS*50

■ Symptom
    When app is on the background, (device is locked, or home screen is on) volume cannot be controlled.

■ Checkpoint
    Check if the product is android or iOS device.

■ Troubleshooting
    When app is on the background (device is locked, home screen is on), volume can be controlled only on few devices. 
    1) For android devices
     - Volume can be controlled with +/- keys for Lollipop (Android 5.x) or higher.
       (Unavailable for Kitcat(Android 4.4.x) or earlier versions)
    2) For iOS devices
     - Volume +/- keys can be used.
       (Speaker volume control on lock screen and alarm screen do not make much difference)