Maybe the apps volume level is low. All you have to do is to go to the Settings and move the app-volume slider to the right or via the app itself.
Go to Settings and tap on Sounds (& notifications). Move the slider under Media volume to the right as much as you want.
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Note: The screenshots are taken using a Sony Xperia M2, running on Android 5.1.1 Lollipop. The instructions mentioned here might slightly vary from one manufacturer to another.
If your device doesn't ring when you receive notifications from an app, it is possible that its notification sounds are disabled. In order to enable them, you should go in the app's notification settings.
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.