If we activate the Mono Audio feature on theOnePlus smartphone, the stereo channels are bundled and played back media, such as music or videos with the same content, is output through the left or right speakers.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  System
Step 3:  Tap on  Accessibility
Step 4:  Enable or disable  Mono audio
  1. Open the Settings
  2. Tap on System
  3. Tap on Accessibility
  4. Enable or disable Mono audio

OnePlus Instructions