Don't know if there is options somewhere in MIUI to do this easy way, but you can change the max audio bitrate by editing file '"/system/etc/media_profiles.xml" (you need root access)...
find these text :
<AudioEncoderCap name="aac" enabled="true"
minBitRate="8000" maxBitRate="96000"
minSampleRate="8000" maxSampleRate="48000"
minChannels="1" maxChannels="6" />
it's almost at the bottom of the file, and
change maxBitRate to "156000", save the file and reboot and try shooting video.
It worked for me.