If the app is the hangup, you might need to wait for Sonos to fix it in a month or two. 

Make sure you have Bravia Sync enabled in the TV settings and run TV Setup in the Sonos app.

Hi - Bravia Sync won't turnoff and where is the tv set up in the app as I can't find it

Disconnect the Arc’s HDMI cable from the TV. In the Sonos App, go to System Settings (gear icon) and select the room associated with the Arc. Scroll down to the “Home Theater” section and choose “TV Setup”. Follow the instructions.

Thanks for this but the app doesn’t even show the room choice

Have you added the Arc to your system yet? Do you see the Arc anywhere in the Sonos app under System Settings?

Yes the app shows my arc and I can go through the process of setting it up but it can’t find my internet and tries to connect through Sonos’s temporary internet. It then plays a few chimes and notes and states it is finalising the connection then says there's a problem and to try again. I’ve tried different HDMI cables as advised by PC World but that didbn’t work either.

Don’t worry about the HDMI connection to your TV right now. Just focus on getting the Arc added to your system. Are you able to temporarily wire the Arc to your router with an ethernet cable just for the setup process?

OK - I'll try the ethernet into the router but I don’t hold much hope as again, ive tried this already. Let me give it a go

Tried it bit still no joy - keep getting the same message that it can’t finalise the connection.

I think I have the same problem. Bought a second Arc with HDMI attachment to SONY TV and despite trying several times - including wiring the Arc to another SONOS amp,)  I cannot get  SONOS to let me finish registering the new one. It will always commence the start up process (including on one occasion   an attempt to start the trueplay ) and on another it gave me the chance to connect via my wifi network via the router. But then everything sudenly stops and becomes ‘a problem’ and therefore registration is never completed. One clue from diagnostics is that all my kit is on the SONOS mesh whereas the only invitation I get from the new Arc is to agree to join my own wifi network. Also only one other SONOS speaker is wired via switch to the router. I just hope this is SONOS problem but I would welcome learning more about your experiences with the new Arc.  

Ok the arc is now registered on the app and has connected to the internet. Still can’t connect to tv as it states there is interference from an HDMI device and to unplug all home theatre products. Done that and it now says there is an update for the arc but can’t download it as there is an error code 1002 so still not fully connected 

I have received the same error code a couple of times. The ‘fix it’ offer in the dialog doesn’t work although the latest controller update notes for IOS suggest this has been resolved. I cannot get any pc Windows controller access at all now which was worked perfectly on S2 before all this went haywire.