Surround sound and Line in via Sonos Amp

Can the ARC's optical input be used like a line-in? Meaning available to be played on any speaker? 
I have a 3.5mm to optical cable from my old Chromecast Audio, that would be neat.

Not sure what you mean. There’s no optical input on the Arc. There is an adapter, and yes, any input to that adapter that matches a codec that Sonos can interpret would play on the Arc, using that adapter. 

For what it’s worth, ARC is Audio Return Channel, Arc is a speaker.

Sorry for the misspelling, I'm talking about the Arc speaker as I had heard it has an optical adapter.  Sounds like that would go to the only HDMI port on the Arc, which needs to go to our TV.  I'm still waiting on the Arc and sub (both backordered), so for now the Amp is connected to the TV via HDMI. 

My issue is all our records/turntable are in my living room/Home theater, and it sucks to have to buy a port to be able to isolate a line in, because when my amp is used as surrounds the line in is disabled.  

There another option then the Port, namely the Five, if your interested in just a single line in, which could be used to connect either a pre-amped turntable, or the line level output from your receiver. Bonus is that it’s a great speaker, too. But no, Sonos doesn’t design the TV speakers to have multiple inputs. Perhaps you could connect the receiver/turntable to your TV, if it has analog inputs?

Given that the Arc only has a single HDMI ARC input, if you were to be constantly unplugging the cable from the TV to insert the adapter to the optical input, you’d get tired of the process pretty quickly. 

If the turn table was in another room, that would be another story.  

Here’s another option I’m thinking of.   Since the line-in on the amp gets disabled when those speakers bonded as surrounds, what about the built-in hdmi port on the amp?  Does that remain active?  Perhaps a RCA to HDMI adapter?  It sounds like once the amp is bonded as surrounds to either the arc/beam/playbar it loses the ability to do anything other than act as surrounds.   Very disappointing.

No, all additional inputs on a device dedicated to surround support are turned off. Likely due to the Special nature of the connection required to the front device, but you’d need to talk to a Sonos engineer. It’s always been this way. Feel free to be disappointed, if you feel the need. 

Same problem.  I buy an expensive Amp and figured it could walk (line in) and chew gum (surround) - doesn't even need to be at the same time!  with a simple switch. I AM A SONOS FAN, BUT A MIGHT PISSED OFF SONOS FAN RIGHT NOW.

Here's my gripe with this...
Neither the marketing materials nor the user guide mention this limitation.
I read the user guides on each and every component before buying this system because the marketing material does a very poor job of explaining the technical aspects.

Marketing material doesn’t mention this limitation under Features and specs, nor in the FAQ:
https://www.sonos.com/en-us/shop/amp.html

User guide - no mention here either:
https://www.sonos.com/support/en-us/sonos-user-guide/index.html#t=sonos-user-guide%2Famp%2Famp.htm%23XREF_95431_Amp

Instead, you have to head over to the support site to find this information.  People don't go to support until after they've found the problem.   That's misleading.
https://support.sonos.com/s/article/4836?language=en_US

https://support.sonos.com/s/article/4807?language=en_US

If it has always been that way, then update your documentation on the main web-site.

I’d like an explanation from Engineering as to why the line in must be disabled.  I wouldn’t even care if the surround speakers don’t play the line-in, I just need it out to the rest of my system.   Also, if both amp and arc are hard wired to my network, why must it “bond” wirelessly?

This should be a software switch, where the speakers remain bonded when in surround mode, then switch to the other mode when line is in in use.

HERE - HERE!  Should be a simple switch.  Heck, I would even be OK if I could do it on my phone with a couple steps - I just don’t want to factory reset the amp, press the join button, etc.  The Amp is tucked away behind everything and I have to pull out the the TV, etc. to get at things.

I agree that this feature is not made as clear as it could and should be.

As to the rest, i start from the position that although Sonos undoubtedly get things wrong, they don’t deliberately set out to make things more difficult for their customers when an easy fix is available.

The nature of ‘bonding’ surrounds is that the surrounds are in some sense ‘slaves’ to the home theatre device (e.g. Arc).  The HT device controls the whole show, and the surrounds just play the stream sent by the HT device.  The nature of their connection to the system is different, and that makes getting the sound out of the line-in to the rest of the system impossible.  (Well, maybe nothing is impossible, but this is clearly a major technical challenge at least, or Sonos would have done it.).

One aspect of the different nature of the connection is that the data passes between the units in a HT setup using 5GHz wireless and direct routing, not via the router.  5GHz is used because it has lower latency than 2.4GHz - crucial in a HT setup.  However, I do not believe that is fundamental to the line-in issue.

For similar reasons, on an Amp or Connect:Amp that is being used to drive surround speakers, the subwoofer port has to be disabled. 

I don’t pretend to know the full technical details of this, so there is no point quizzing me further about it.  But the way surrounds connect to the system is fundamentally different from other speakers, so it is not surprising that some behaviours are different.