i have the same sound issue.seems related to the recent releases rom.
this issue appears since two or three releases.
i think we have to wait an update and see if the bug is solved.
don't forget that miui rom on mipad is still in beta
I am a beta tester at en.miui.com and I have had this issue for the last few releases of V6. I, and others, have reported it but we receive no answers there either along with the bluetooth driver issues and the still lack of MIUI functions (alarms, weather and themes) months after release.
In my opinion the Mi Pad is a top notch hardware but the OS/UI/drivers are not ready for release yet. Support is spread too thin and it isn't getting sorted in a timely manner.