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
Mono audio -  Open the  Settings
Mono audio -  Tap on  System
Mono audio -  Tap on  Accessibility
Mono audio -  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