Hi @Michael Greco, welcome to the Sonos Community!

If rebooting your devices resolves the issue, then it’s likely that the issue lies with one of the devices you’re rebooting or the connection between it and Sonos. Instead of rebooting everything at the same time, try rebooting one item at a time then testing to see if that brings audio back. Another option you can try is to re-seat the HDMI cable between the Sonos Arc and TV, or TV and streaming box, to see if that also brings audio back.

You mention powering down with the remote, but generally that only powers down the TV. Are you powering down the Sonos Arc at the wall socket? Also, how are you powering down the AT&T streaming box, does that also power down via the remote? 

If audio only comes back when you reboot the Sonos Arc, I would suggest reaching out to our support team for further assistance, as they have the necessary tools available to troubleshoot this issue with you.

I hope this helps!

This morning I turned on the TV and the Streaming Box using the streaming remote.  The Sonus Arc is always on.

There was no sound through the Arc.

I powered off the TV by unplugging from the power socket.  Waited 5 mins and plugged back in.

Still no sound through the Arc.

I powered off the Streaming device bu unplugging the power cord.  Waited two minutes and plugged back in.

There WAS sound through the Arc.

Does this indicate a problem with the streaming device?

The wiring looks like the following

ATT streaming device - (HDMI connection) - LG TV - (HDMI Arc connection) - Sonus Arc

Perhaps try turning off CEC control on the ATT streaming device and see if things then work for you, the only issue then though is you will probably need to use the TV remote to control volume etc. 

Note you may need to power all the devices off/on again to get the TV/Arc CEC settings to work correctly after switching the protocol off on the ATT streamer.

The HDMI-CEC control was already off

So are you using IR to control your Sonos volume then, rather than CEC? What happens to the TV audio if you disconnect the ATT streaming box, does the TV audio then work okay for you. I’m just wondering if the CEC is actually turned off, despite what the settings are showing to you. If it works without the streaming box connected, it might be worth inserting a CEC-less adapter between the ATT box and the TV perhaps? But see how it goes without it connected first. 

Here’s a link to the CEC-Less adapter I referred to:

https://www.amazon.com/Lindy-HDMI-Adapter-Female-41232/dp/B00DL48KVI

Obviously I cannot guarantee it will resolve your issue, but it’s perhaps worthy of further consideration, particularly if things work okay without the ATT streaming device connected to the TV.

Sonos Arc and LG TVs are the common issue here 

Haven't been able to identify if it's an LG thing or Sonos, but I've had this same issue for over 2 years now (never had this issue when connected to Vizio TV).

Sound works about 80% of the time when I turn on the TV and start watching something. Easiest way for me to "fix" this, is to jump into advanced settings on the TV and toggle on/off Sound Bar Mode Control OR TV Sound Mode Share.

So, let’s look at it this way. 

• Sonos designs their systems to work with a normal CEC (across HDMI-ARC / eARC)
• The Sonos Arc works fine with every other TV manufacturer, including your previous Vizio.

What does logic suggest?

I’d suggest three possibilities. First, there could be some sort of issue with LG’s implementation of CEC on this model of LG. Second could be a hardware fault on your Arc. Third would be some sort of network issue in your LG communicating with the Sonos Arc. My bet is on the first option, but if you were to submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, you could probably rule out options two and three. Or figure out a fix to it.

Agreed on the potential issues and that it is likely the first one.

Next time this happens I will submit a system diagnostic and call support. Thank you!

I seem to be able to resolve my problem by turning my power strip off and back on.  All three devices (LG TV, Sonus ARC, and AT&T streaming device) are connected to the power strip.

I use my AT&T remote to control the streaming device, TV and Sonus volume. 

Using my AT&T remote I turnoff  the TV and streaming device at night.  I wake up the next morning ans turn the TV and streaming device on using the AT&T remote….and there is no sound coming from the ARC.

The way for me to get it to work again is by powering off the power strip.

The frustrating part is the system worked perfectly for more than a year.  I suseect that a SW upgrade to one of the devices as disrupting the operation.