Tried using an optic cable and configured the HDMI input on the AV Receiver as well as the audio settings on the decoder. Finally, it's working.
Thank you for your kind input Ivan.
Thank you very much indeed for such a comprehensive and useful response, I will certainly try using an optical cable. Hopefully that will resolve the problem. As I mentioned, I'm getting video (via HDMI IN 3) but no sound. I'm using a projector via HDMI OUT 1 (eARC) port.