Volume Keys not recognized in Android

If I press the volume keys on an Android device while running a Qt app, the device does not recognize it as a volume change, the volume stays the same and the volume info bar is not shown.

Steps to reproduce

• start a Qt-App
• while running the app, press volume buttons

expected behaviour:

• volume changes
• volume bar is displayed

actual behaviour:

• nothing happens
  (- volume stays the same
• volume bar is not displayed)