Playing Apple Music directly from the Sonos app streams at 256 kbps AAC. And, as far as I know, AirPlay 2 is about the same quality.

If you have your music library stored on a computer or NAS, you can add your music library to the Sonos app and stream your lossless ALAC or FLAC files at a higher quality (up to 24-bit/48kHz).

Read more here:

https://support.sonos.com/en-us/article/add-your-music-library-to-sonos

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

Thanks. I see around 1Mb/s on my router from Internet to the Sonos device when playing a ALAC file that was uploaded to Apple Match, that’s 4x the 256kbs you quoted? I have found the Apple Music documentation on Sonos website, but was hoping for some more detail.

The Apple Music app on iOS via Airplay 2 is lossy, Airplay can send CD quality but the Apple Music App does not.

Not related to Sonos but has some info…

https://audiophilestyle.com/ca/bits-and-bytes/apple-music-lossless-mess-part-2-airplay-r1026/

Thank you @GuitarSuperstar and @Belly M for info. This is really academic/curiosity to me, as my ears are not perfect 

I originally ripped CDs to ALAC, so I wouldn’t have to do it again. I had a NAS, and worked well. Now looking to move forward with the times/declutter and use Apple Music to store my collection to provide a more flexible solution, eg remote access to Music Library.

I was reading link below, which suggests its only possible to stream Lossless using Apple Music App on Apple or Android device, connected using headphones or powered speakers.

https://support.apple.com/en-gb/HT212183

So, I’m assuming if there is wireless link (Airplay/bluetooth) it will be the 256k AAC.

I’m still curious why I see 500-1,000kbs/ on router though, I would expect to see ~300kbs for a 256 stream.

If I had an Apple TV 4K, connected to a Sonos Beam, and I launch Apple Musics on my iPhone, Airplay to ATV, would it stream in lossless/Atmos, ora do I have to launch the music from the ATV Music app directly?

I’m slightly over 50% sure you need to start music from the Apple TV device itself. There’s limited bandwidth available for the AirPlay 2 stream from your iPhone to your Apple TV (although Apple might, and you’d need to check with them) know enough to switch from an actual stream to handling it on the Apple TV anyway...but that’s an internal to Apple thing, which I’m not certain about. Hence my hedging above. 

You're right, it's an Apple thing, I'll check with them. Thanks,
But I have read somewhere else in the meanwhile that it should work: start a song from iPhone, Airplay to ATV connected to Beam Gen2 and you should have both lossless and Atmos (at your choice). 

I'll update once I have more news.

I’d love to know more about how Apple handles stuff started on the phone, and handed off to the Apple TV….I’ve just never bothered to research it, mostly because it’s just as easy for me to start playback from the TV, rather than digging in to my pocket.

Yea, I’m lazy. Why do you ask? :)

I don't have a TV. The Apple TV would be connected to a projector, to have an image I'd have to pull down a screen which hangs from the ceiling and it's usually rolled up and I don't want to do that every time I want to listen to a good quality track. 

AirPlay 2 is limited to stereo, it doesn’t send a 5.1 signal to anything non-Apple, including a Sonos Beam. Hopefully, your projector has the electronics in it to create an ARC or eARC signal, which you’d then use to connect to the Beam to get that 5.1 or Atmos signal. 

But if you’re just listening to music, a stereo signal is all you need, and AirPlay 2 is a fine way to send that to the Beam (not the Apple TV). No need for the projector for that to work.