If you want to mute the sound when typing on the keyboard, then you need:
1. Open the settings and go to the "Language and input" section, which is located in the "Personal data" settings block.
2. Next, go to the “Virtual Keyboard” section.
3. You will see a list of keyboards installed on your device. Select the keyboard that you use for typing and go to the settings of the selected keyboard. Options will be available here to mute keyboard vibration and sound. In order to turn off the sound when typing on the keyboard, move the corresponding switch to the “Off” position.