Sonos supports FLAC but the maximum sample rate can only be 48kHz. So convert the files to 48kHz/24-bit FLAC using audio software like Audacity.

Read more about what audio formats are supported for a Sonos music library here:

https://support.sonos.com/en-us/article/supported-audio-formats-for-sonos-music-library

All a higher sample rate gives you is the ability to capture frequencies higher than a human being can hear.  It's pure snake oil, unless you are playing music for your dog.  Studies show any quality differences are due to better mastering, not higher sample rates.  So resample at 48 kHz and be confident you aren't missing any quality.

Does the sound quality diminish though by reducing the Khz size from 96 to 48?

Not unless you are a dog, or a bat.  The sampling theorem used in audio states that all of the signal under one half the sampling rate is captured and reproduced exactly the same as the original analog signal, with no loss.  1/2 of 48 is 24 kHz.  The human ear cannot hear anything over 20 kHz, and if you are out of your teens, it’s far less than 20 kHz.  

By the way, if you are ever perusing audiophile sites, and you see anything about “jaggies” or “stairsteps”, or “digital is an estimate, and a higher sample rate gets you closer to the original” run away.  They are either ignorant, or lying to you, and neither one is to your benefit.  

There are no peer reviewed studies proving that anything beyond the CD sample rate of 44.1KHz can be perceived by the human listener.

In the studio, during production, there are benefits when the producer uses higher sample rates while processing the session, but there is no audible benefit using these higher rates for distribution to the retail listener. For anal producers, they should be operating at sample rates higher than 192KHz. 192K is lame if you are being anal.

A minor historic detail: SONOS has used 24bit internal processing since inception in 2005.

Aside from the higher sampling rates being inaudible snake oil, they would add to the communications load between speakers and services which would lead to increased dropouts.

It would be interesting to see some numbers from folks in heavily congested WiFi locations on the reliability of the various options, to include ATMOS when channel utilization is 60% or higher.

I have my local music at 16 / 44.1 as I, or a few friends I’ve asked,  can not hear any difference between that and the higher resolution ones.

For a music lover the only thing to understand about audio formats is whether they deliver better sound to the extent that humans can hear, and as said by others here, Hi Res does not offer that. Indeed, even Apple AAC, that is a format that compresses CD files, offers outcomes that are audibly the same as CD quality, something that is vouched for by the head of Apple Music.

Doing the one time exercise of resampling the Hi Res music you have, also as suggested, is all you need to do to hear music of the quality that is on those Hi Res files, via Sonos.

Music sold as “Hi-Res” can often sound better than the standard issue, but this is due to more careful processing, beginning with higher quality master tapes, not some sort of higher playback bit rate.

Some less than ethical producers will (at a premium charge) simply resample a regular CD in a “Hi-Res” format. At best, this release will sound no better than the original CD.

Also: none of the above gains will be lost via downsampling to a file that Sonos can play. And of course, if the quoted above has not been done, ask the question why you have paid extra for the Hi Res files in the first place.

Thank you for all your feedback, that’s really interesting to hear your views and will have a think before buying any more Hi-Res files.

I always had the understanding that AAC was better in sound quality than CD or am I wrong? Like if you buy a 256kbps Mastered for iTunes album of The Beatles Abbey Road that it would be better sounding than your old CD album you purchased 20 years ago?

It can’t be. AAC is lossy. It throws data away.

(Some might argue that this makes it sound ‘nicer’ but it’s not what the artist intended.)

That’s a different matter, independent of the format issue. Remastering should obviously (we hope) improve on the original, sometimes dramatically.

I have recently remastered albums on CD, so if I wanted to rip those songs from the CD into digital format, what would be the best format to rip them in and using what software?

I have always ripped my CD’s into Apple iTunes m4a format but is this losing the quality that is on the CD?

Since you have both versions, you can easily check for yourself if you hear any difference between the CD and the rip. If you do not, all else is moot.

It depends whether you rip to ALAC or AAC. Both use .m4a files. ALAC is lossless.

There are so many such out there on iTunes, that I suspect that this is just Apple marketing to allow people to think that compressed AAC is as good as CD quality. I have never found any difference between known performances on CDs v lossless CD rips v this remastered for iTunes thing. One just has to be make sure that sound level differences are not fooling one into thinking otherwise, and if that is the case, fixing that difference just needs a nudge of the volume level controls.

Moot or not, if one is going to all the trouble to rip one’s CD collection (IIRC it took me 3 weeks, all day every day, back in 2007) it is simply crazy to use a lossy storage format.

Then either the remastering was a waste of effort or something else is amiss. Remasters are often, but not always, audibly different from the original issues. Even if the mix is similar the higher frequencies are usually cleaner.

And from someone that spent a decade down the rabbit hole of audiophilia: I find that even a streaming source such as Spotify gives me the same sound quality I used to get from my high end CD/SACD players, now sold. 

Change the speakers or where they are placed in the room, and you will hear audible differences, even day and night ones. Do anything else in a typical domestic listening environment, and you are wasting time and money better spent on music listening and many other things.

Try premium headphones, from a premium DAC. 

The reason why I got into Hi-Res music is because I bought a Sony NW-A306 Walkman with Sony WH-1000XM4 headphones and that I now keep my entire music library on.

I think the ultimate goal was to listen to classic albums in Hi-Res with my Sony gear. I don’t use iTunes that often anymore and only have a very old iPod Nano that I use to listen to music in my car.

Though a lot of my music is still m4a and iTunes registered but always use the Sonos app now to listen to these files.

All the more reason to rip CDs to lossless, either FLAC or ALAC 16bit/44.1kHz. The NW-A306 will talk LDAC to your XM4s, hopefully without sample rate conversion, and hopefully at 909kbps.

With the room acoustics taken out of the frame, that would make sense if listening via headphones was my cup of tea. It isn’t so I will not go down that rabbit hole now, and create dissatisfaction where there is none today.

I rip to FLAC as it is likely to have the longest lifetime as a supported format.

Once ripped in FLAC I can easily convert to anything else.

Which conversion I did for the car with its limited storage and weak sound system. It holds a lot more in the MP3 format and on that setup you can’t hear the difference. I also stuff some MP3s on my laptop to listen to in hotel rooms.

If ripping, lossless is the obvious route. My question was to the OP question on whether 256 AAC, if mastered for iTunes and purchased would sound better than the CD - why not listen to both and trust your ears to tell you? As well as for albums ripped already - the same test would be valid regardless of whether AAC or ALAC has been used. When I stopped buying CDs, I moved to buying such 256 AAC albums from iTunes and found SQ to be just as good. As I now find Spotify to be, having stopped buying music completely.

Of course if the OP is using premium headphones and the like, differences I do not hear may well be audible and then those would be relevant in deciding how to proceed.