Hi @prezzo
Thanks for your post!
Alternatively, assuming that you want the audio coming from the Arduino to play directly on your Sonos system, rather than the Sonos system getting access to the same library as the Arduino, you would need a Sonos device that accepts Line In like your ZP90, but that works on S2.
The cheapest way to do this with a new product would be to buy an Era 100 and a Line-In adaptor for it. Connect the Arduino to the adaptor with a 3.5mm to 3.5mm stereo cable, and Line In will show up in the Sonos app as a source for all your Sonos devices (not just the Era 100).
You may be able to find a cheaper, second-hand option, of course - these could be a Port, Play:5 (Gen2) or a Connect (Gen 2). The Port and Connect would both need a RCA-3.5mm stereo cable instead.
I hope this helps.