@logemann
Hallo Marc and welcome to the german community,
sorry, Port does not have a ground connection pin and I was told that we probably don’t need GND with a preamped signal.
Check if your turntable has a built-in preamp like the Pro-Ject T1 Phono SB Turntable. If your turntable does not have a built-in phono preamp, you will need to first connect your turntable to a phono preamp, and then connect the phono preamp to Sonos Port (Line in).
On a phono preamp you will usually find a ground connection pin.
Please check:
Support article: Listen to your turntable on Sonos
In our english community I found experienced users talking about GND here. I’d suggest asking there.
Further help from project-audio:
Turntable Electrical Hum Guide (PDF, EN)
Plattenspieler elektronischer Brumm Leitfaden (DE)
https://www.project-audio.com/en/faq/
Best regards
Nils