The "Mute all sounds" function can be found in the Listening section of the LG smartphones' Help menu, and when activated, all sounds from the device are muted.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  General
Step 3:  Tap on  Accessibility
Step 4:  Tap on  Hearing
Step 5:  Enable or Disable  Mute all sounds
Step 6:  Confirm with  Turn on
  1. Open the Settings
  2. Tap on General
  3. Tap on Accessibility
  4. Tap on Hearing
  5. Enable or Disable Mute all sounds
  6. Confirm with Turn on

LG Instructions