Xiaomi Redmi 4. How to increase incoming call voice level.

First, you need to root your device.

Second, you need to open /system/etc/mixer_path_mtp.xml file for edit, find the following line and increase value (to 90 in my example). Do not increase value higher than 95 or you may break your speaker.

<path name="voice-handset">
  <ctl name="RX1 Digital Volume" value="90"/>
</path>