This thread on the kodi forums is the same issue on CB2 HD from DVB-T2 "surround sound"
Playing UK Freeview HD TV content from a mediaportal server, with audio set to “passthrough” and connected to amp via HDMI
OSMC shows 2 channel audio on screen,the amplifier plays the surround channels as the front left and right
As the original file is over 2GB I have split the file with tssplitter to get to a more manageable size and hopefully not have any copyright complaints
mediainfo shows
General
ID : 4 (0x4)
Complete name : C:\temp\split-1\Marvel's Agents of S.H.I.E.L.D. - 2016-03-19.TSSplit.20-120.ts
Format : MPEG-TS
File size : 20.0 MiB
Duration : 47s 670ms
Overall bit rate mode : Variable
Overall bit rate : 3 514 Kbps
Video
ID : 48 (0x30)
Menu ID : 137 (0x89)
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4
Format settings, CABAC : Yes
Format settings, ReFrames : 4 frames
Codec ID : 27
Duration : 46s 840ms
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate : 25.000 fps
Standard : Component
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Color range : Limited
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709
Audio #1
ID : 64 (0x40)
Menu ID : 137 (0x89)
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Muxing mode : LATM
Codec ID : 17
Duration : 47s 680ms
Bit rate mode : Variable
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Frame rate : 46.875 fps (1024 spf)
Compression mode : Lossy
Delay relative to video : -1s 85ms
Language : English
descriptor_tag_extension : 6
Audio #2
ID : 65 (0x41)
Menu ID : 137 (0x89)
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : HE-AAC / LC
Muxing mode : LATM
Codec ID : 17
Duration : 47s 573ms
Bit rate mode : Variable
Channel(s) : 1 channel
Channel positions : Front: C
Sampling rate : 48.0 KHz / 24.0 KHz
Frame rate : 23.438 fps (1024 spf)
Compression mode : Lossy
Delay relative to video : -1s 0ms
Language : English
Language, more info : Visual impaired commentary
descriptor_tag_extension : 6
Text
ID : 80 (0x50)
Menu ID : 137 (0x89)
Format : DVB Subtitle
Codec ID : 6
Duration : 45s 238ms
Delay relative to video : 2s 60ms
The same issue is not experienced on kodibuntu or kodi on Windows.