Ian,
I haven't looked at Sonos' blurb on what you can do with Alexa, lately, but can you give it voice instructions to play a particular album from a particular folder on your music library? Its folder structure is several layers deep. I'm not at all interested in streaming music services, just playing my own music, and accessing internet radio. I don't currently use Sonos speakers, its just the method of getting the music from my NAS to my amp, and the only actual Sonos component is the Connect box. As I'm unimpressed with the Sonos Android app, and its perverse layout (the PC one is far more readable, but why aren't they identical in appearance?) I'd like a less fussy way to play the music I want.
No, it will only locate tunes by voice via a streaming service (because the magic cloud that processes your commands knows nothing – nor has access to – your NAS). You'd have to use the app. The iOS Sonos app looks fine and will index any music on your NAS. I can also play Spotify, Amazon Music etc. via AirPlay to get around the restriction that ties you a premium service for direct streaming. But again, unless the tunes are online (or you use something like iTunes Match) the voice command won't work other than for skip, volume up and down, etc.
To be honest, that's probably fine for me, I'm still prevaricating over Spotify Premium, a state of indecision that may last for another couple of decades. I mostly bought it for the speakers to replace my ancient stereogram. Far better than faffing about with bluetooth.
It's great for internet radio, I just have to say
Alexa, play [station name], and job done. There's probably much more stuff, I'm sure. She certainly doesn't know about train status though, because I just asked her. Southern should give her a job, she's ideally qualified.