Lip-sync issue over ARC seems to be OK now. I may be speaking too soon and may jinx it lol, but hopefully it's OK
I'm not sure what sorted it, it may be a combination or just a single setting tweak.
I've been in the settings on my old 5.1 system trying to get it to match-up for days with no joy.
Then Steve's comment got me thinking, he's tried new soundbars and receivers, all probably 4K capable and so he probably has to enable HDMI Deep Colour on the TV for HDMI2(ARC). I thought I'd try sticking that on despite the lack of 4K/HDR passthrough on the old 5.1 setup I am testing.
I think this may have helped as things seemed closer, that or I toggled something else somehwere too. The Xbox seemed a lot closer but still quite poor and unwatchable really.
I tried the PS4 Pro with Uncharted 4 and it seemed to be working OK over ARC. I tried the PS4 YouTube app and it seemed perfect.
I tried C8's Netflix app and that seemed good. I tried C8's YouTube app and that was oddy still off. I checked the C8 audio adjustments were at default values and I then popped back into the 5.1 system, it was set back to 0ms for a lipsync setting. I scrolled through 100s of values and got to Auto, figured I'd try that.
It worked. Even though 0ms was good for C8's Netflix and good for PS4, for whatever reason the C8's YouTube app doesn't like it - auto seems to have sorted it.
I am not 100% sure if enabling HDMI Deep Colour for the ARC HDMI2 port had any real signficance or not, but I think the Xbox improved a bit after this.
Though improved, there was still a noticable delay with the Xbox set to Dolby or DTS. Stereo works, but it's not exactly what anyone would really want.
Assuming it stays working at least those 4x full-bandwidth HDMI ports can be put to good use and passthrough other devices to a soundbar.
It's a shame about the Xbox One X having a Bitstream delay over ARC. My thinking was the Xbox could stay connected directly to the C8 and keep both SDR and HDR in Game picture mode, and no longer use the console as a 4K disc player - as it does not seem likely tbat the Blu-ray Xbox app is getting a DV update. Some games do support Atmos, if they are lossy DD+ then I guess this could ARC passthroigh. Either way, the main appeal was just not having to mess with settings, and a Dolby Vision 4K UHD would be connected directly to to the soundbar.