Hi Paul.R.S, your issue with ARC is interesting where it switches itself to TV Speaker after selecting Receiver (HDMI). You may already done this but first make sure settings are correct with the receiver and TV for ARC.
On the receiver settings, enable HDMI Control and ARC and on TV settings, enable AnyNet+ (which should be enabled by default) and select sound Receiver (HDMI).
Connect HDMI cable from receiver HDMI Out 1 (ARC) to TV HDMI 4 (ARC).
And before re-testing ARC, remove optical cable from TV to receiver as this might confuse the receiver and/or TV. If ARC is working the receiver should auto switch to TV Audio by default.
Some suggestion to test ARC:
- try using a HDMI with ethernet cable from TV to receiver and re-test ARC
- remove all HDMI input cables from receiver and TV (except HDMI cable for ARC) and re-test ARC
- disable all blue-tooth devices, phones, receiver, TV, headphones, etc.. and check Samsung Multi-Output Audio is not enabled...General > Accessibility > Multi-Output Audio (note: if this is enabled then something is still connected) and re-test ARC
- If still not able to get ARC working. remove HDMI cable from TV HDMI 4 (ARC) input, power down TV and receiver, remove TV and receiver power cords, wait for about 15-30 minutes. Plug in the receiver and TV power cords. Turn on TV and receiver and then connect HDMI cable to TV HDMI 4 (ARC). Go into Samsung menu and make sure AnyNet+ is enabled and sound Receiver (HDMI) is selected. Also go into receiver menu and make sure HDMI Control and ARC is enabled. Now select the TV app to play and hopefully ARC will auto switch receiver to TV Audio.
Disclaimer for testing 4: some device does not recommend removing or connecting HDMI cable when device is power on, so try at your own risk. Just note I have done this (twice) with no ill effect when Receiver (HDMI) option disappear from the sound menu.