How to disable Notification Sounds and System Sounds on an Android Smartphone?

Instructions for Samsung Galaxy

To disable system sounds:

Go to Settings (gear icon)
Tap on Sounds and vibration
Disable system sounds – move sliders from right to left (the slider icons will be greyed out).

System sounds are disabled now on your Android smartphone.

To turn off notifications for selected applications:

Go to Settings (gear icon)
Tan on Notifications
Tap on More
Turn off notifications for chosen apps- move sliders from right to left (the slider icons will be greyed out).

Notifications for selected apps are disabled now on your Android smartphone.

To disable all notifications sounds:
1.   Go to Settings (gear icon)
2.   Tap on Sounds and vibration
3.   Tap on Notification sound
4.   Select “Silent”.
Notification sounds are disabled now on your Android smartphone.

Find more detailed instructions here:
How to disable Notification Sounds and System Sounds on an Android Smartphone