Cannot play FLAC at bitrate 172k or 192k despite sonos indicating it supports up to 1536k

You’re confusing sampling rate and bitrate.

1536kbps is the maximum bitrate, for PCM 16-bit 48kHz.

Sonos doesn’t support a sampling rate above 48kHz. No human can hear ultrasonics, by definition. 

Thanks for your response. Sorry about the confusion, yes I meant 192kbps and 176.4kbps.

They should play but somehow the S2 app indicates that it can't.

No, you didn’t. You meant 192kHz and 176.4kHz sampling rates.

The controller message was that the file “is encoded at unsupported sample rate 192000 Hz”.

No, they shouldn’t play. As I said above, Sonos doesn’t support a sampling rate above 48kHz.

In fact, if you scroll down in the same support site you linked to:
 

Does this mean that all S2 fans waiting anxiously to play their hi res files are going to be disappointed even after Sonos finally makes that much hyped feature available?

Disappointment requires an expectation, and Sonos have not promised anything specific on hires.

Sure but I thought that hi res was commonly understood to be at least 96/24. 

Amazon has conveniently changed that to mean CD quality, but that is a recent "innovation".

So-called ‘hi res’ is anything beyond Red Book. 

https://en.wikipedia.org/wiki/High-resolution_audio

a term for audio files with greater than 44.1 kHz sample rate or higher than 16-bit audio bit depth. It commonly refers to 96 or 192 kHz sample rates. However, there also exist 44.1 kHz/24-bit, 48 kHz/24-bit and 88.2 kHz/24-bit recordings that are labeled HD Audio.

But wiki itself says that 96 or 192 khz sample rates are typical, being commonly understood to be hi res.

So all the kerfuffle about S2 is still not going to meet the hi res needs as commonly understood.

Of course I write this listening to 320 kbps lossy Spotify where the sq is compromised by something else entirely - fluctuations in sound levels from one track to the next. Addressing that would be meaningful progress in my book, instead of this hi res red herring that does nothing audible as an improvement to the listening experience.