Messages are acoustically suppressed when the "Do not disturb" mode is activated. If you still want to hear them, you can add an exception in the LG device settings. You can choose between Nobody, All, Contacts only or Favourites only.

Note: The Only with priority mode must be activated.

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  Messages from
Step 6:  Choose between  No one ,  Everyone ,  Contacts  and  Favorites
  1. Open the Settings
  2. Tap on Sound
  3. Tap on Do not disturb
  4. Tap on Select priorities
  5. Tap on Messages from
  6. Choose between No oneEveryone, Contacts and Favorites

LG Instructions