Apparently, the LG C9 does support Atmos(TrueHD)/DTS HD passthrough over eARC but only from HDMI input, not for apps like Plex.
I have asked for this feature to LG support over the phone myself (and I encourage others to do the same).
There is also a thread here: https://lgcommunity.us.com/discussion/4633/c9-earc-not-passing-truehd-atmos-through-to-receiver
