By default, LG device mutes app notifications when in "Do Not Disturb" mode. App notifications are shown on the display but cannot be heard. In the settings, individual apps can be added to the exception. For example, you can hear WhatsApp notifications acoustically even when the mode is enabled.

Note: Priority Only mode must be selected.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  Sound
Step 3:  Tap on  Do not disturb
Step 4:  Tap on  Select priorities
Step 5:  Tap on  App notifications
Step 6:  Enable or Disable desired  Apps
  1. Open the Settings
  2. Tap on Sound
  3. Tap on Do not disturb
  4. Tap on Select priorities
  5. Tap on App notifications
  6. Enable or Disable desired Apps

LG Instructions