Hi everyone, after searching high and low, I managed to set up the Sonos Arc as part of a 5.1 system to play perfect with the OLED65B9PLA and also using the Zappiti HDR 4K Pro as a player. I thought to share these settings with any Google searchers.
OLED65B9PLA Settings
Connect the Arc using the cable included to HDMI 2, which is the eArc port. Turn the Arc on.
The TV should, itself, now output sound to the Arc. Go to sound settings on the TV and under the sound output options for external speaker, select Passthrough (which will send the signal without any decoding from the device you are playing from, in this case the Zappiti, to the Arc). Also turn on eArc.
In the more sound options, you can also set sound input for the device connected to each HDMI port. I have set all these to bitstream.
With these settings, using the TV:
- You should be able to play any sound from any app on the TV itself perfectly. Try Atmos from Netflix, it should show up on the Sonos app properly, or even from the Apple TV app.
- You should be able to play DD+, DD, TrueHD or Atmos from any connected device providing you have set up the sound out correctly on that device.
Zappiti 4K
This is a great device and I have used it since ditching discs; I populated my content using MakeMKV/Xmedia. I have connected this to HDMI1 and use it to play most my content. I have used it for DD+, DD, TrueHD and Atmos (TrueHD); all work perfect.
Anyway, the options are really simple. In sound, set HDMI sound to RAW, not auto. Auto provides very mixed results in terms of sound output. RAW always delivers the right sound to the TV which just passes through to the Arc.
The other options are not relevant; I have no idea what HMDI Rx is for but it appears to make no difference to sound on the eARC port.
With regards to LPCM, I thought I had to change the setting to output LPCM and wasted hours figuring why it didn't work. In summary, Zappiti does not transcode anything, so you cant (for example) output DTS-HD as LPCM and then play it on the Arc 
Sorry, I thought you could Anyway, what you have to do is create an LPCM track for the film and then simply output it as RAW (it as above) and it plays perfect.
Put another way, Zappiti sends LPCM to the Arc to play perfectly, but there actually has to be an LPCM track on the video to send. Sound options stay as RAW whatever you are playing.
Hope this helps someone!!
