When the app is on in the background, (device is locked, or home screen is on) volume cannot be controlled.
Check if the product is android or iOS device.
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)