I’m not familiar with the Sonos products. That said, the 3.5mm to 3.5mm stereo cable would seem to be the right approach. How is the turntable connected? Does it have a built-in phono stage, in which case it should be connected to the Sprout’s analog in (another 3.5mm jack)? If not,it should be connected to the Sprout’s MM turntable input.
What happens when you use the 3.5mm cable? Do other sources work, such as bluetooth?