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.