I don’t have one to check but is that the Sonos adapter?

Many of the ones folks try are designed to convert the wrong direction.

Even if you get one converting in the correct direction it still must have an internal chip supported by the Sonos firmware. Don’t ask, I have no clue what that would be.

I’d also flip that switch back to phono then back once or twice. It’s in the correct position now, but I’ve seen them get ‘crusty’ before. 

ADC. Most ‘third party’ adapters are DAC. 

Thanks very much for your responses. The line in adapter is not Sonos brand- I’ve just ordered one online. Hopefully that fixes it!