This happens if you have connected your TV to a 5.1 channel sound bar or home theatre system.
When it is on S/PDIF audio mode in your Mi Box, you can only control volume from your home theatre system. I encountered this while playing 5.1 surround content on Prime Video and Plex.
abembe
Since the Pie update, this has happened a few times. Reboot
TheDoctorMunster
You can try connecting TV's coaxial cable to your Receiver or Home Theater. I have connected in this way and it's ok.
Aboelhmd
Go Mibox setting, Change digital audio format to PCM
This happens if you have connected your TV to a 5.1 channel sound bar or home theatre system.
When it is on S/PDIF audio mode in your Mi Box, you can only control volume from your home theatre system. I encountered this while playing 5.1 surround content on Prime Video and Plex.