[CyberVillain]

The library view reports both files as 5.1 Dolby, one passthroughs AC3 to the receiver the other does not.

version: Windows v17.6

Adjust display refresh rate: on start /stop
Sync playback to display: off (This one will disable AC3 since Kodi needs to resample the audio to match new framerate)

Passthough: enabled
passthough device: WASAPI HDMI
transcodig: off

DTS capable: on
DTS-HD: on

Receiver: Denon X3300 fed through HDMI

[CyberVillain]

I had only enabled AC3 but missed AC3 Plus (ops)

[lovemachine]

EDIT: Didn't see you already solved you're problem.

Something to try: In Player Settings->Language>, de-select Prefer Default Audio Settings.  If there are 2 audio streams in the file, Kodi may be selecting the first, default audio stream that is Stereo.  

If that doesn't work, try posting a log as described in the main help page to get more help.

[CyberVillain]

EDIT: Didn't see you already solved you're problem.

Something to try: In Player Settings->Language>, de-select Prefer Default Audio Settings.  If there are 2 audio streams in the file, Kodi may be selecting the first, default audio stream that is Stereo.  

If that doesn't work, try posting a log as described in the main help page to get more help.

 Thanks yeah. Stupid me