Well buried but:

https://support.sonos.com/en-us/article/play-line-in-on-your-sonos-amp

Note that Line-In is disabled when Amp or Connect:Amp is being used to power surround speakers in a Sonos home theater.

What will work is another Sonos device that offers Line-In. A Era 100 and line-in dongle is the least expensive, a Port is the slickest option.

Thanks. Not what I hoped, but thanks anyway.

I wonder why, if the Arc is not in play (turned off), the Amp would not be receiving a signal from the soundbar, opening up the Line-in. Sounds somewhat reasonable, so it must be wrong.

If the CONNECT:AMP is ‘bonded’ as a surround driver, that is when, as @Stanley_4 states, the line in is turned off. Due to the nature of the connection used for surround sound, which is essentially one way. 
 

When/if the CONNECT:AMP is grouped, instead, the line in is not turned off, since it is now a peer device with the soundbar, and no longer in that parent/child relationship. 

I own the Amp, not the CONNECT:AMP.  My Amp is grouped with Arc and Sub 3. I am still wondering what happens to Line In on the Amp when the Arc is “off” and not in play. 

The Amp and the CONNECT:AMP are the same in this respect, since they run the same software. 
 

When the Sonos device is set up as a driver for surrounds, there is no way to ‘turn off’ the Arc. Any stream sent to the ‘room’ activates all speakers in that Sonos ‘room’, although you can adjust relative volumes. 
 

Grouped, however, is different, since they aren’t ‘bonded’ as surround speakers. In that case, as has been discussed before, they’re considered peer devices, the connection is different, and the line in is available. But you don’t get the surround channel from the Arc at that point. You get a stereo representation of all channels the Arc is receiving. 

SInce the TV can be turned on at any time (and any reasonable person would expect the audio to come on with it), the Arc is never “not in play” and therefore the bonded one-way connection between the Arc and surrounds (which takes time to configure and get the handshaking done) is persistent, not dynamic.

I’m not clear having read this a couple of times just how the amp is connected to the Arc… both group and surround have been mentioned and both have very different implications. Any chance of a screen shot of how it looks in the app? The advice might be a little more targeted once that’s clear… 

What is unclear?  The OP stated that they are using the Sonos Amp as surrounds.

My Sonos Amp is happily pumping surround sound from my Arc.

When AMP is setup as surrounds, there is only one Room that includes both the soundbar and AMP. Even when adding a SUB, or two, there is still only one Room. In order to then use the AMP as a stereo amplifier, it would need to be broken out as a separate Room.

Little complications: What if the soundbar is currently playing as a stereo Room and is the coordinator of a Group? What happens to the Group if the AMP is brought back to the soundbar. Actually, this is currently dealt with. (Try deleting the Group Coordinator — the first member of a Group) After removal from the Group, AMP would then need to be bonded to the soundbar as the surround amplifier. Further details might involve SUB’s and Trueplay.

Bottom line: there are a lot of messy details involved. How many customers would benefit from this capability? Does this number justify the development and maintenance efforts required to support the feature, efforts that could be spent on more popular features?

OK, this is not going to happen. My thanks for everyone’s replies.

This means, of course, that I do not have to purchase the WiiM, and I can keep my cash to see what’s inside the “Arc Ultra.”