First, it's a work in progress. Second, I had better luck using devices of the same brand. Third, some content sent back out from the HDTV in this manner is still restrictive. IOW, you won't always get what you want (e.g. 5.1, etc.).
Maybe it's what you wrote though that has me slightly confused. Your speakers should be hooked up to your Panasonic unit and not to anything else. Digital optical is not capable of ARC, FWIW. Also, *all* devices must support ARC for this to work, not just one with ARC-HDMI and the other, only plain HDMI.
It would be helpful if you were to be more explicit about precisely how *everything* is hooked up currently(?) That way we can further deduce a solution. For example:
LG55LA7400 (HDMI output from ARC port) ---> (HDMI ARC input) Panasonic unit
I have a new LG55LA7400 and a Panasonic SC-HTB20 home theater unit. When I hook up the speakers (ARC HDMI out) to ARC HDMI on the TV and set the TV for external speaker ARC/digital optical I don't get any sound. I have also tried using an optical digital cable between them, but still no sound. My TV input is over-the-air. The set seems to recognize the speakers, but says no input signal? Any thoughts.