go back to miui and make sure that microphone actually works.
if it works, then make sure you are on the firmware that dev recommended.
I made some tests. Recordings with the Recorder app are loud enough. Perfect! Doing it with apps like Whatsapp or other messengers its very quiet. So i think it is a software problem and can be fixed by an update.
Hopefully the developers find a solution. Thank you for this ROM
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Another Problem:
Sound for typing at the Keyboard are turned off. But they i have sound while typing anyway. Mute-mode helps.
Maybe for the next update?