What does your setup look like? do you have a receiver that the Mi box is plugged into or is the Mi Box going right into the TV?
If the Mi Box is outputting 5.1 audio that would mean it is the Dolby Digital+ track being output from Netflix. There would have to be something that is in your setup that is decoding the dolby digital track. If it is a receiver then you can check if it has HDMI CEC and the Mi Box would be able to adjust the volume of that device instead of its own volume.
The only reason I can think when 5.1 audio volume "won't change" is when pass-through is in effect (then the volume is controlled through the audio output device, not the video output device). You can disable pass-through on device settings