My experience with ARC
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