Your means is that your voice is bad when you call others. Can you try to replace a Earpiece Speaker Receiver Replacement Parts for Xiaomi Mi4? Dragon-tt.com supplied any xiaomi repair parts.
To my opinion black xiaomi 4 is more beautiful than white mi4. It is worth to wait. I believe it will be available soon, because black full screen is on sale in dragon-tt.com.
All the keys have a limited life span! Too many times pressing will make your mi3 damaged. Why not come to take a look at the Dragon-T when you need some spare parts.
I suggest you go to dragon-t which provide original xiaomi repair parts including LCD screen assembly touch screen and any other flex cable.
You can also refer others suggests in
http://xiaomi.eu/community/threads/mi3-touch-screen-broken.23634/
When someone call you your phone will sound or vibrate。Your band will vibrate after about 20 seconds(How long intervals you can set according your needed.)
I think no original new main board is available.May be I can find disassemble one from smartphone. I suggest you should buy a new smartphone instead of main board.