If it supports ARC then you should be able to connect the Mi Box to the TV and have it feed sound back to the DVD player/surround thing.
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According to the link it does.
So make sure DVD player is connected to the ARC port on TV side (there is only one and it will be marked).
Make sure HDMI-CEC etc is enabled on TV and anything related on DVD player side.
Sound should then come down from the TV and any devices connected to it through the sound system.
MiBox connected to one of the other tv ports will also feed through ARC channel.