I am having the same problem. Did you ever resolve it?
I just tried to install a Panasonic SC-HTB10 sound bar on my Samsung UN55C800XF using the HDMI/ARC connector. The Panasonic documentation says this will work on any ARC-compatible TV but it doesn't work in my case. I've tried every setting on both the tv and the sound bar but no luck. Everything works if I use the digital (optical) audio connector but I don't want to use this because it means adding another remote controller to the mess on the coffee table. Has anyone had success connecting external speakers via the HDMI/ARC connector?