Problem solved
Hey all,
I bought a LG 55LW5700 55" LED 3D TV last week, attempted to set up my home system the same way cbrian3 did, and ran into the same problem.
The solution is to change the settings on the TV for how each HDMI input (listed separately) is passed on to the optical output, from PCM to AC3 (5.1). I had to get a technician to come out to do this, as it requires a special remote and password that LG only gives technicians. He had to do it via a menu screen that looks completely different from the ones that come up for the tv remote. Why isn't this available to me? He shrugged.
LG phone support couldn't explain what was going on, as initially I had a problem getting any audio to come through, but at least they were quick to set up a technician visit, covered by warranty. At some point when I'm in a zen mood and can handle inanity I'll call LG back to ask for an explanation as to why they didn't think consumers could handle the huge responsibility of changing the output signal settings. Can you imagine if you wanted to upgrade your sources from stereo to 5.1 one-by-one over time, requiring an LG-authorized technician visit each time?!? Thankfully I am all done with that.
There doesn't appear to be a lag on the audio after the video on the tv's "normal" video mode, but it was noticable on "cinema." I didn't notice it while gaming in "gaming" mode, but it's hard to tell. Either way, audio lag with this setup is preferable to video lag with a setup running hdmi through the receiver to the tv.