I doubt that will work.  It looks like chromecast supports Hi Res, but you would need your TV to accept HI Res over chromecast.  Then the TV needs to pass hi res over optical, which I am pretty sure optical cable can’t handle (It doesn’t handle hi res over home theatre channels, not sure about PCM stereo) and then the playbar needs to play hi res over optical, which I doubt it’s built the handle.

The Playbar doesn't support HiRes (I.e. 24-bit) audio. I can't see any way around that. 

Human hearing is unable to distinguish ‘Hi Res’ from 16/44 CD quality. I can’t see any way around that either.

Good point.  I meant that there was no technical means to play HiRes on the Playbar, not that that actually mattered!

Thanks, everyone!  I know that I probably can’t even hear the difference between 320kbps and CD-quality, much less hi-res.  And I don’t pretend to understand how Sonos works. But I was and still am curious about what it means that the Playbar can’t support Hi-Res audio.  I understand that when it is serving as both speaker and processor of music received via the Sonos app.  But if it’s getting its audio from the TV (which may be getting its programming via cable or via Chromecast) and not from the Sonos app, what is it about the speaker that can’t process the sound that is delivered to it?   

In short, the Playbar hardware. Even if the S/PDIF was able to deliver 24-bit PCM in the first place, the audio pipeline in the Playbar almost certainly wouldn’t be able to deal with it, at least for the TV input.

One way would be upgrading Playbar to S2 as this article suggests it is possible:

https://www.techradar.com/uk/news/sonos-speakers-finally-get-hi-res-audio-streaming
 

“The ability to support 24-bit streaming first came to compatible Sonos products with the introduction of the S2 app; these include the Sonos One, One SL, Five, and Play:5 speakers, Sonos Arc, Beam, Playbar and Playbase soundbars, and the brand's portable speakers, the Sonos Move and the Sonos Roam.

The discontinued Play:1 and Play:3 speakers don't support 24-bit streaming unfortunately (as well as any Sonos speakers that still use the S1 app), though you'll still be able to stream in 16-bit via Qobuz.”

I know hi-res isn’t supported on the Playbar for Amazon Music, but maybe Quboz is different. I’ve never used Quboz so don’t know for certain.

That’s not how the OP was looking to get hi res audio though.  The user wanted to go phone/tablet → chromecast ->TV → optical cable → playbar.