This question was asked and answered in the owner's thread, and the answer is still: run everything through your AVR, not the TV! ARC will support 5.1 for apps (and live TV), but it would still be better to use optical (Toslink) out because ARC is problematic.
Yamaha RX-A2050 AVR [v1.81], LG 65EF9500 4K OLED [v4.26.00]
5.0.2 set-up: (4) Polk Monitor 10B's, (2) Polk Monitor 4's, Polk CS300 center
B&O TX2 Turntable, Nakamichi Cassette Deck 1
Pioneer CLD-99 LD, Panasonic DMP-UB900 UHD [v1.80]