Aha, the service manual is located here, and it has a separate schematic page for the audio output block. Page 44 of this PDF:

https://ia800505.us.archive.org/12/items/LG_55LE5400/LG_55LE5400.pdf

So what's the best spot on that schematic to add a couple of RCA jacks for line-out? I don't comprehend everything that's happening in that schematic. Any ideas?