Welcome to the forum!
From what I've read, that LG
will pass 5.1 audio from an HDMI input through the optical output. However, it appears that you need to go into the service menu to enable that. I haven't found a simple step-by-step yet in my quick searching, so I suggest that you look online and see if you can find it - if you can't, just come back and we'll help you.
EDIT: Okay. Try going to
this thread at AVS. Post 948 looks like it has good info, but some others might, too. Also,
here is a link to service menu codes for different types of remotes.