Question:

 How can I fix the issue of no notification sound from certain apps?

Answer:

Ensure notification sounds are turned on:

Navigate to Settings > Sounds and vibration > Notification sound

Select a preferred sound from the list

Tap Back > Volume > Adjust Notifications slider to desired level

Check device sound settings:

Go to Settings > Sounds and vibration > Sound Mode > Select 'Sound'

Disable Do not disturb mode:

Swipe down from the top of the screen > Tap 'Do not disturb' icon (glows blue when active)