@AE-Alex: Hi, I tested this for you with my Vero 4k and an AVR Sony STR-DN840 using SPDIF and a TOSLINK cable.
Here all works fine with your settings but I’m already using Debian Stretch [TESTING] OSMC images for Debian Stretch which will come as an update hopefully the next days.
What very wonders me that you even cannot hear the GUI menu sounds which make me believe that there is some physical issue between the Vero 4k and your Spectral BRA1 like
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Cable issue? Light intensity might not be high enough using the Vero but just above operational threshold when using the TV?
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Dirt in the SPDIF port of the Vero 4k? Very carefully you can push the protection flap of the Vero’s SPDIF port and look inside whether there is some foreign particle behind the flap.
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A general incompatibility? Never heard about such issue using SPDIF. Is your Spectral furniture thing on latest greatest firmware?
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A defect SPDIF port of the Vero? No idea how to verify this without another SPDIF receiver and second proved working TOSLINK cable.