Not sure this makes any sense, at least the way I read it.

What Sonos speakers are you talking about? There are very few Sonos device that take a line level input through an analog input port, and no Sonos speakers that connect to speaker wires. Only the CONNECT:AMP and Sonos Amp are used to connect to speaker wires to send a signal to third party speakers ( Sonance, or other companies)

Sonos devices contain a small computer in them, that take an internet connection and reach out to get and play an external stream. In your case, it sounds like that internet connection is supplied by the LeGrand. 

What are you trying to achieve here?

Ignore the above.  Your statement, “use a new Sonos amp to power two in-ceiling speakers”, is perfectly clear. 

What isn’t clear is why -- or whether -- the installer gave you Ethernet connectors to your speaker wires.  Ethernet cables are typically used to daisy-chain one digital zone amp to the next in a system, not to connect speakers to an amp.  Even in a huge Euro DIN-Rail panel box, the speaker wires are individual, “wire looking” wires, screwed individually into captive holes in the mini-amps.

Sooo…..  Your best bet to get help here is to specify your model numbers AND take a photo for folks to look at.  The interesting part to the Sonos-heads here, will be whether you ought to do as you plan, namely divorce a single pair of speakers from your Legrand Nuvo or OnQ system and hook them up to a Sonos Amp, or whether you should hook up a Sonos Port to one of the analog inputs on your system and let the system route and amplify the single Sonos zone to the whole house.

If you cannot or will not do that, well… Are you sure about Ethernet connectors to your speakers?  Double-check: look at a photo on the Internet for an Ethernet “RJ45” connector (typically clear polycarbonate plastic with a tab on one side), versus a photo for a commercial slide-in speaker connector (typically green or black ABS plastic, with screw-down holes on one end for the wires and “wiper” contacts on the other end, to be pushed straight into the rear panel of the amp). If you are desperate, you can pull down a speaker grille and unscrew the 4 hold-down screws and their associated rotating tabs which hold the speaker up in the drywall, and look at the wire connections there.  You should have 2 regular old-fashioned *wires* per speaker (maybe 4, for a single-point stereo speaker) and they will head into some kind of cable sheath.  Find the other end of that same-looking cable and you’re on your way!

(N.B. The only time I have seen RJ45 Ethernet connectors to speaker wires was in a Bang&Olufsen system, where “Power Link” line-level audio is being sent over those little tiny networking wires, to an amplified speaker at the other end.  But this is not how a Legrand system is wired. ((IME:-)) )

[Apologies for being so wordy & responding to my own post, but the forum won’t let me edit now.]

Even more important might be what controllers you are using to drive your Legrand Nuvo (or OnQ) system today: If the original installer set up (say) an entire Control4 system, then you probably want to keep it and leave your speakers on the Nuvo -- because Control4 has drivers for Nuvo and Sonos too, very neatly integrated and ready to roll!  But if you love the Sonos app and have only wall keypads for the Nuvo, an argument could be made for slapping iPad Mini’s up on the walls and switching 1-for-1, Nuvo zones to Sonos Amp’s!