Just got my c350 today but it is the 55 inch. It has an eArc port and it has Dolby Atmos support. Plugged in my Sonos Arc into the eArc port and the Sonos app shows Dolby Atmos sound. Tested with Hbo Max app.
Returned the 55 after 1 day. The price is great, but you get what you pay for. The brightness leakage around the edges was too distracting, particularly with sports. Coming from a 120Hz screen the difference to 60Hz was noticable. The colors were also hard to get consistent from channel to channel. As a streaming tv, it is very nice. Nothing negative about the FireTV and it supported all my existing audio devices. But had to spend a bit more to get a series 6 instead. If you just need a tv, the c359 is great. If you are used to better TVs, you'll notice the lower quality easily.