I had recently bought a Mi Box 4K directly from a Mi outlet. While the setting up was straight forward I was struggling to figure out why I was not getting any audio on the TV. Even at full volume the audio was extremely low. I then connected an AV cable (red, white, yellow connectors) from the headphones port of the Mi Box to the TV but there was still no audio. I then realised after some reading online that it was an optical port and it needed an optical Toslink cable. I kept searching online for a solution and finally found someone else having the same problem and guidance was provided. As it goes, the remote of the Mi Box has a volume control too which controls the volume of the TV. Apparently, this volume control by default was at "0" !!! The moment I increased the volume by pressing the "+" key, the TV responded well. I placed the volume from the Mi Box remote to maximum and then controlled the volume with the TV remote.
In the absence of such information in the very basic user manual, it can become very challenging for some one who is not very tech savvy with today's gadgets. Just a simple indication in the Manual would have been appreciable. Even some YouTube videos that I watched did not address this matter. I was almost concluding that the Box or the HDMI cable was defective !
Anyway, I wanted to share my experience as if someone else has a similar issue will know how to get around the problem.