Does that particular turntable have a built in pre-amp? If so, you need to switch in on, to the ‘Line’ position.

If it doesn’t, you’d need to get an external pre-amp to put between the turntable and the Sonos. There are dozens available on Amazon, for instance, all relatively inexpensive. 

Page 4 of the manual for that turntable suggests it does not have a built in pre-amp. 
 

https://www.project-audio.com/wp-content/uploads/2018/03/Manual_Debut-Carbon-DC.pdf

Maybe try this...Goto "Settings/System//Sonos Five Room Name]” and set the Line-In "source level” to 10 and see if that improves things.

Have tested that, but I felt it wasn’t enough improvement. It’s a quite expencive turntable, and I’m hoping for really quality sound without any fuzz!

Sonos says it has a built-in preamp, so thats quite dissapointing....

The “Sonos edition” of that turntable contains a phono preamp (unsurprisingly), which obviously needs to be turned on. It looks like the regular version doesn’t include a preamp.

A phono signal that hasn’t been through a preamp isn’t just at a very low level, it also lacks RIAA equalisation. It’ll sound horrid.

Hey Ratty! Found the correct button, and now everything is just perfect

It was the Phono preamp with a button hidden under the turntable.