I'm sure I'm not the only person out there who is using YoutubeTV with a newer Samsung TV (with the One Remote) that has the built-in YTTV client. I like having a Chromecast, as I cast from my PC's and tablets to the TV a lot, and I have the Chromecast showing our photo library continuously. Because I use YTTV, I wanted the TV to turn on to the Chromecast input, so that I could pick YTTV or whatever app or input I wanted, without having the OTA TV blaring in the background.
I did some setup work on mine today and I have a suggestion for anyone who has a similar setup, and wants volume control with the Sonos to work for the native OTA Samsung TV tuner, a Blu-Ray player (or any other HDMI device), and the Chromecast. If you do a "Setup TV" from the Samsung menu, and you have the Chromecast on HDMI 1 and a Blu-Ray on HDMI 2, the TV will automatically recognize the Chromecast and the Blu-Ray. Generally, it will set up the Samsung One Remote to work with the Blu-Ray, but not with the Chromecast, because the Chromecast lacks any kind of remote control, RF or IR. This will cause problems later, because when you follow the procedure in the Sonos FAQ to get it to work with the One Remote as a "Home Theater System", you cannot link the Sonos to the Chromecast as the FAQ describes, because in won't show up in the "Universal Remote Setup" device list, as it has no remote. This means you can't control the volume while using the Chromecast. There's no problem linking the Sonos to the Blu-Ray, because it can be controlled by the One Remote, and shows up in the "Universal Remote Setup" device list.
The solution is to create a dummy device that's also on HDMI 1 in the "Universal Remote Setup". I found creating a new device and selecting the Roku box from the OTT device choices worked fine. You can rename it "Chromecast" if you like, but the cool thing is that because the nice "Chromecast" icon that the Samsung creates is also associated to HDMI 1, you can use it and now have the volume controlled, too. You could also accomplish the same thing by unplugging the Chromecast during the Samsung TV setup, then plugging it in when it's done, then creating a dummy HDMI 1 device as above, but you wont get the nice Chromecast icon.