I had similar problem as mentioned in:

except that I was using an AMP instead of a Beam. I followed the steps to unplug both TV and AMP from power and disconnect HDMI cable at both ends. Next I reconnected HDMI cable to both again followed by powering up TV first then AMP, then retried the TV Setup function in the Sonos App for the Home Theater. This time it immediately found the ARC between the Samsung TV and the AMP.

Problem resolved. I never determined why it lost the ARC connection as it had been working for at least a year...