I'm going to read the manual at http://www.lg.com/uk/support/support-product/lg-49UH661V
And we are done.
"This function is available on certain models or in certain countries only."
LG opted to not document this connection but on many sets it's for use with a soundbar with volume controlled by the sound bar.
Since the speakers have no volume control, this seems a poor match to this TV.
I've got an LG 49UH661V tv and while I was trying to connect Genelec 6010A speakers through headphone output, the volume level is way too high even when volume level = 1.
Also tried with 8020C speakers, and the speakers own volume control in the front seems "over sensitive". And third test was with headphones. Also way too high volume level, and really no change in the volume level when increased from 1 to 15!
Is this really a LG feature, or a bug in the component, or what?