Make sure you use updated firmware and the correct FTMC for your hardware.
If you have a different device make sure it runs Nougat firmware and use Kodi 17.3 to get working HD audio pass-through.
Edit: I see you use Realtek Beelink, so FTMC won't have pass-through for you as it is not designed for Realtek devices.
You need to set up FTMC/Kodi 17.3 with an external stock video player then to get pass-through.
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