Sounds like you want to group your home theatre room with another room you have setup with Sonos.

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

That said, you do not need to connect your laptop to the TV to play your iTunes.  If the files are stored locally on your laptop, you can setup a shared library for Sonos to access the music files locally.  If your streaming from Apple music services, then the speakers can also do that directly.

No, speakers can only belong to one room at a time.

You could remove the 300s as surrounds each time, and set them up as a stereo pair for music playback, but that process is both onerous, and loses any TruePlay settings each time, if you have them. Most people just set the surrounds to ‘full’ rather than ‘ambient’ for music playback when they’re connected as surround speakers. Then they operate as a normal pair of stereo speakers, rather than support speakers for the Sonos sound bar. 

I was hoping this was not so.  It seems like I picked the wrong speakers for my needs.

I truly did not expect Sonos to be so inflexible in their configurations and unable to support multiple configurations for the same room.

Luckily I am still able to return the speakers and get the LG S95QRs. They support multiple sound fields.

Your use case seems very specific to me. You could Airplay to Sonos from your Mac or your phone and you can ad Apple Music to your Sonos system. Why is connecting a Mac via HDMI necessary for you?

Hi 106rallye,

I use my Mac for my Plex Media Server and VLC app to watch live tv stations and sports in addition to listening to my iTunes music which is my CD collection of 35 years.

Also, my wife uses the system when I’m not at home and likes everything easy to use. Before I purchased the Sonos, she had finally become comfortable with the setup and liked everything working from one remote.

Hi Airgetlam,

if I understand your response correctly, I can add a new room (like music room) and assign the speakers to the music room (which would just be arc+era300+era300}. 
 

this would allow me to use the living room when I am playing movies and streaming Disney or Netflix and then select music room when I play my iTunes or watch a tv show that is just broadcasting stereo.

We use the term ‘group’, rather than ‘assign’, but essentially, yes. And when ‘grouped’ , the single One will play all data, since it’s a single device. The content is codec doesn’t matter, that single One will play a mono signal, as if it were the only thing connected to the TV. It essentially downsamples whatever the Arc is playing to either mono (for one speaker) or stereo (for a stereo pair). All of this is ‘automatic’, nothing for you to do other than to group the two rooms when you want. 

Thank you Airgetlam,

I’ll give this a try.  If I understand, as l will create a music room, add the same speakers I have in the living room, and then group the two rooms together. 
 

 If it works, I won’t need to return the Sonos speakers.

Yes, you can always ‘group’ two Sonos rooms, which will play in sync, for music streaming. The only time there would be a delay between the two rooms is when the TV itself is the source device. 

@Deaconblue I know for you this would count as a work around, and it would need for your wife to use her phone instead of a remote, but you could ad Plex to Sonos and play your music that way.

I’m not an Apple Music/iTunes user myself, but I seem to remember you can also upload your music to Apple and listen to it that way - Apple Music can also be added to Sonos. See https://support.apple.com/en-us/HT204926 I do think you need to keep a separate copy of your music because I remember reading Apple might replace your music with their copies.

Hi 106rallye,

Thank you for your input.  I tried Sonos support again.

After an hour and 40 minute call with Sonos support (resetting speakers and powering off our modem - basically setting back to factory defaults and start the set up from beginning) we were able to get the arc + era 300s to play the iTunes music with surround turned on.

I need to turn Pass Through off on Digital Sound Out and use the PCM setting on the LG TV sound settings so Sonos recognizes the signal as Stereo PCM to use all 5 speakers.

This solution is acceptable. I have decided not to return the speakers. I’m just hoping I do not regret my decision.

If it works for you then it’s OK. To me it is strange that a stereo TV source should play on all speakers, since in my experience all stereo TV sourced material at best only get a slight pseudo-surround form the rear speakers, but I might not really get what you’ve achieved - not owning an LG TV, nor an Arc.