I have this set up - a Sonos Connect:Amp in the kitchen, rigged up to ceiling-mounted speakers. I plug a 24" LCD TV from its 3.5mm headphone socket into the line in on the Amp, and it works fine. In fact, it's ideal - when the TV is on, the line in on the amp takes precedence over everything else, so you don't need to do anything to the amp when starting the TV up, even if it was playing music from the queue before.
You can control the line out levels on the TV by using the TV remote, but it's not very effective (ie the level of line out needs to be pretty high on the TV to be at a volume on the Sonos commensurate with other music playback etc) - the best thing to do is to use the Sonos app, or in our case, because the TV is in front of the amp, just reach around and prod the volume buttons.
Or use an Amazon Echo and say "Alexa, kitchen volume 6" 🙂