Your Apple TV goes into any HDMI port. The Sonos Arc must be connected to the port labeled ARC or eARC. 

Thank you. I know that’s very basic and I appreciate the answer. I can move forward. 
The way it’s going at the moment I haven’t gotten sound from the back speakers, pair of Era 100. During the test I heard sound from 1 speaker when I did True test. At the moment I have a flashing red light from the other speaker. 
 

So, there are several things I’d recommend.

Reboot that red blinking speaker, by unplugging it for the wall for two minutes, then plug it back in. You want to get it to a steady white(ish) light. If it’s blinking red, a TruePlay won’t work, as the speaker isn’t connected to your Arc. 

Make sure there’s no potential wifi interference , particularly in the 5Ghz range after than, near either speaker. Note that all interference doesn’t have to come from a WiFi enabled device. 

Make sure the source you’re watching is at least Dolby Digital, and shows up as such in the Sonos app’s ’now playing’ screen. 

Basic questions are good. There are a lot of ‘remove this particular variable to hone in on the real issue’ that has to go on. No need to apologize ;)

Oh, it may not hurt to remove those speakers as surrounds, and a stereo pair to get them into a ‘normal’ state, then try adding them back as surrounds. Frankly, if you’re going that far, it wouldn’t hurt to reboot the Arc, and your router too. 

Thanks for the assistance! I will try this. It just requires a bit of patience! I’m looking forward to having everything working properly. 

There’s a lot of smart and experienced folks here who try to help. If you run into something else, just ask. Not being Sonos employees, we can’t help with failed hardware, but that’s extremely rare and unlikely. There’s always Sonos Support , but given it’s New Years, I’m not sure if they’re manning the phones or not. 

Make sure the TV is set to these settings:

HDMI Input Audio Format: Bitstream

Digital Sound Output: Pass Through

eARC Support: On

Set the Apple TV Audio Format settings to this:

Change Format: OFF

Dolby Atmos: ON

Hi. This is a great community website. I have my Era 100 speakers working. But have some issues with the TV settings such as you have outlined. I have the Apple Format settings as you mentioned. The LG C3 TV sound settings have some things you mentioned there as greyed out. Not sure why but still searching. Thank you for your help. 

Often, TVs ‘grey out’ settings that can’t be changed depending on the data be sent through (what you’re watching). Make sure the source is an Atmos signal, and all of those settings should be available. Don’t try to change them while watching something in stereo. 

If that doesn’t work, you may need to contact LG’s customer service. 

Perfect. Those are great suggestions I didn’t know about. 

Hi. I have another question with the above setup. Everything has been working well. But, tonight my Sonos app shows my Arc is on silence for 1 program. 
There is no sound on one show only in Prime going through on my Apple TV.  I can go to other programs on Apple or to my streaming box and they all have sound. 
So unplugged my Apple TV box and plugged it back in and now there’s sound. 
Can you think of a reason this happens? Actually this is the 2nd time it’s happened. 
 

As a guess, the Apple TV or the TV set didn’t initialize the CEC ARC stream properly, and wasn’t sending correct data to the Sonos. It took a reboot to get that stream to initialize properly. Software is sometimes sensitive to power surges. You may benefit from putting all three devices on a surge protector device. 

I will give that a try. 
Many thanks.