I have a very similar problem with my mi4w since the middle of october. My voice sounds robotic and wavy. Before problem came I used xiaomi.eu's MIUI 8.2 then 8.5 stable that worked fine. Tried a lot of rom versions, but it's only a temporary solution. Works fine for 1 or 2 days then problem comes back. No matters it's MIUI 8.X or 9 version. Replaced SIM. Changed network mode from 3G to 2G and back. Tried to turn fluence mode off. Replaced microphone flex cable. Nothing helped. Other mobile service provider's SIM card seems fine.
Last week I had a promising 2 days short test with xiaomi.eu's latest Android 4.4 based 18.104.22.168 stable rom. But when switched back to MIUI 9 weekly, problem comes back. Now I'm trying again with 22.214.171.124.
If this would work, my question is what is the difference between this 4.4 based rom and the 6.0 based roms, that can cause this sound problem?
Or anybody have any soluition for this?
Thanks a lot!
I have a painfully fast answer for the above: after 2 days, 4.4 based rom has the same sound problem. Interesting new thing is outgoing calls produce this issue. Incoming calls seems OK. Tried two different SIMs from the same provider. I have no idea what more can I do. :-(