I would like to know that too...
If nothing, we will have to find help on miui forums ... 🙂
edit : Could someone try doing this, to try to fix mic problem ?
Disassemble Phone.apk and navigate to res/values/bools.xml to change:
<bool name="send_mic_mute_to_AudioManager">false</bool>
->
<bool name="send_mic_mute_to_AudioManager">true</bool>