To adjust the volume level of all notifications:

You can modify the notification sound for all apps, or assign a distinctive sound to notifications from a specific app, replacing the default.
Do one of the following:

Tap the category for enabled notifications, then tap Sound and select a unique ringtone, or set it to none.
If an app's notifications are silent but you want to hear them:
To let an app's notifications be heard when Do Not Disturb is on:


To temporarily silence notifications from all apps, use Do Not Disturb.
If you want an app's notifications to be less intrusive, you can categorize them:
Option 1: On a notification
Option 2: In Settings
Go to Settings > Sound & vibration > Do Not Disturb.
Or, in quick settings, touch & hold 
Select what to allow or block:
To hide the icons for silent notifications from all apps:

Restart the phone to apply the new settings.
You won't see the icon in the status bar for any silent notifications. When you swipe the status bar down, the icon appears in the pull-down along with the notification.