I have My Ultimo4K Plugged into my TV via HDMI.
My TV is connected to My AVR via Audio return channel (ARC).
My TV will pass 5.1 Dolby Digital / 5.1 DTS audio from my Ultimo4k to the AVR, It will also pass Dolby Digital 5.1 from The Netflix, Amazon Prime, NowTV apps on my TV if 5.1 sound is present.
What ARC can not do is pass HD audio such as PCM 5.1/7.1, TrueHD, DTS-HD MA, this is a limitation of HDCP and bandwidth, in such a case you need to connect directly to an AVR via HDMI (or you can use multi channel analogue for PCM only).