I don’t understand why your home theater setup mutes when tv has been turned off. 
If you use an IR remote causing this, it will be expected behavior imo that the setup stays muted even when starting a music source. Just an autoplay playback with preset volume imo would do so. 

 All I do is turn off the TV with the Comcast remote.  Even when I ask Alexa to turn off the TV the Arc goes Mute.  When I turn on the TV the Arc HT comes out of Mute.  Only when I start some music does it remain muted.  

 If I use Sonos Voice the Arc comes out of mute.  Not so when music is started with the app.

 I don’t mind the muting when the TV is off.  I only think starting music should take the Arc out of mute.

@MoPac 

Imo it‘s not expected behavior that Arc gets muted by turning off the tv. Maybe there’s a setting on your tv for that or it’s doing that by default for connected audio devices. 
However… once it’s muted it sounds logical to me that it stays muted besides an autoplay volume preset is used or you unmute manually. 

I’ve not ever heard of that before, where powering off the TV mutes the connected Home Theatre (Sonos Arc) - I presume it’s the TV’s CEC implementation which is likely causing that issue. I’m curious as to what the make/model of the TV is? It might be worth researching the TV online, just to see if any others may have reported the matter. I would also check to see if there are any firmware updates from the manufacturer that might (hopefully) resolve the issue.

 TV is a Sony Bravia from 3 years ago.  It’s up to date.  The issue is not so much the TV muting the Arc.  I just wonder why Sonos would not automatically take the speaker your trying to play to out of Mute if sees the speaker in that condition.  The assumption would be the users intent is to hear the music so let’s take it out of Mute.

Do you have a “ smart” or programmable remote, which has been programmed to reduce volume to zero when switching off the tv? 

It’s normal behaviour for Sonos not to change the volume of a speaker/group when starting a music stream, so you have to look to the “turn tv off” process to see why the audio volume changes.  

You say “I just wonder why Sonos would not automatically take the speaker your trying to play to out of Mute if sees the speaker in that condition.” What volume do you expect to automatically be set, in your wondering? 

My “Hey Google, good morning” routine sets a specific volume, in my case to be able to hear the news at different volume then the music my kids played the evening before.

You could do this too, if you turn off the TV at night and want music on a set volume the next morning.

 Don’t have a smart remote.  The Comcast remote is mapped to the TV volume.

 The audio volume does not change.  It just goes into Mute when the TV is turned off ( green LED ).

 When music is started it would be OK to use the last volume that was used when the TV was on.  The only time I hear large volume differences is when I change from a stereo music source to a Dolby Atmos music source.  Atmos is louder.

@MoPac 

You could run a test by disabling cec functionality (Bravia sync) on your tv. If Arc then doesn’t mute the test is speaking for itself. Otherwise as ultima ratio you can try a reset on your Arc or before doing that call Sonos support to check a diagnostic. 

 I purchased a Beam Gen2 for the bedroom.  So now both my Arc and the Beam Gen2 drop to 0 Volume when the TV is turned OFF.  Both TVs are Sony, but the Sony in the bedroom is older and only has ARC not eARC.

 It would be nice if Sonos soundbars had a default volume setting that would be applied when the TV is turned on.

The default volume setting when TV is turned on is the previous set volume.

I have not heard of the TV muting the soundbar whenever the TV is turned off. Common denominator for you is Sony, suggesting Sony send a signal by CEC over ARC/eARC to mute soundbar as part of turning off TV. I have not experienced such behaviour with Samsung or LG TVs.

My Bravia with Google TV certainly sends mute over HDMI-CEC to my AVR when the TV is powered off and I don’t have TV (hdmi eArc) selected as the AVR input.

If I am streaming music from my Nas, Qobuz or Internet radio while the TV is on, for example, the TV will display what is playing. Switching the TV off, the AVR front panel will flash up ‘TV: Mute’ but as it is an input not in use it has no impact on my listening. I suspect the reason the front panel shows the mute message is my AVR supports 4 zones and can assign different inputs to different zones.

For most AVRs and Soundbar this is a non-issue because when they go into standby they power down most circuits and if you connect later to play music via an app/bluetooth/chromecast/airplay they switch on and restore the volume to it’s previous pre-mute value.

If I power my Sony TV on later it reads the volume value from my AVR rather than setting the AVR volume to some level the TV decides.

I think Sonos devices are different in that they never power down that far, so where as other devices will un-mute and return to previous volume as they switch on, Sonos don’t.

The idle power consumption of Sonos devices is typically higher than other manufacturers standby + network/bluetooth enabled values, who have a meets eu regulation on/off style mode and a user choice network connected higher power standby mode.

The difference is they need to power up from standby so take longer to playback, whereas Sonos is always on so can start playback quicker.

If my held for moderation post appears this can be removed.

TL:DR version, which will hopefully be filter friendly

My Bravia Google TV sends hdmi-cec mute on power off. My AVR clearly shows it receives an input mute.

Why? I’d guess it’s part of Sony’s fix for their Android audio implementation which pops and clicks on codec changes or stream stops.

When the TV switches on and connects to the (e)arc audio device it reads the volume level from the arc/earc audio device. Tv Speaker volume and e(arc) device volume are maintained separately in the TV.

For most AVR/Soundbars, standby switches most of the device off. When they power on they clear mute and set volume back to the previous level or a default level.

This is a problem for Sonos which behaves differently because their products never go into standby and they don’t track separate input volumes. My AVR mutes the tv (eArc) source input without affecting other source inputs.