Since ColorOS 7 a touch sound is played when touching, for example the navigation buttons, but if this is not desired on the Oppo smartphone, the sound for touch can be disabled in the sound settings.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Sound & vibration
Step 3:  Enable or Disable  Touch sounds
  1. Open the Settings
  2. Tap on Sound & vibration
  3. Enable or Disable Touch sounds

Oppo Instructions