Use the tape out into the Port input. 
 

@notquitemytempo 

Depends on your intention to use it… whether to get a signal from the audiosystem via port into your Sonos system like @nik9669a discribed (but as I can see that just could be radio or something like cd connected to aux in) or to play some Sonos sources on the speakers that are connected to the audiosystem. In that case (what is the main usecase of a Sonos port) you would have to connect ports analogue audio out to the aux in of the audiosystem. Depending on Sonos settings you will be able to control volume via Sonos or via the audiosystem. 

Hi @Schlumpf 

This is a hi-fi system with integrated vinyl turntable / FM Tuner / cassette player. I want to be able to play my vinyls on my entire sound system in multiroom. Sorry, I didn't specify…

Thanks for the answer, I will try it !

@notquitemytempo 

Thanks for clarification… so the tape out will be your friend. But if you like you could use both options and also can connect ports out to the aux in. Just if the speakers of that system are quite worth it. 

Okay, thank you very much!

Precisely no, the speakers of the hi-fi system are very old that's why I would like to use only my Sonos system.

I'm going to test all this!