I connected the Mi Box to two TVs, one LG and the other one a Sony TV, they both have Dolby Digital decoder built-in and it's active for HDMI input so when the Mi Box detects this it shows "SPDIF passthrough" below the auto detection option.
The issue is no sound was heard when I played back something, it doesn't matter what app is, no sound comes out of the TV's internal speakers, I had to switch it manually to "PCM" in order to get sound.
As far as I know, any audio should be heard, all the codecs should be reproduced except for Dolby TrueHD and DTS-HD Master Audio, which can be decoded so far ONLY by an AV receiver, other codecs like Dolby Digital Plus, Standard Dolby Digital, AAC and LPCM should be downmixed and played back with no problems.
Are you having the same issue?
if you doesn't connect your Mibox to TV with SPDIF cable (only HDMI) and check in settings "SPDIF passthrough" you will get no sound. HDMI already support DD and DTS so you doesn't need SPDIF if you can coonnect TVBOX with hdmi directly.