Airplay and iMac and Sonos 1 SL

I recently bought a SONOS 1 SL:

https://www.sonos.com/en/shop/one-sl

Idea was to be able to use it as a desktop speaker off my own iMac with AirPlay.

What works:

1. I can AirPlay to the speaker from my phone
2. I can AirPlay to the speaker from my (company owned) MacBook Pro
3. I can SEE the speaker from my iMac (meaning it appears as an available sound output device with the AirPlay icon next to it)

What does not work: I cannot get any sound at all out of the speaker when I connect to it from my iMac

My iMac (and MacBook) are running SONOMA 14.0 and are on the same network as the SONOS 1

I have restarted everything !

Any ideas ?