I usually don’t recommend a line in connected to a variable output like a headphone jack, it adds to many variables to the equation.

What Sonos device do you have this line in connected to? Is that Sonos device set up as a surround speaker or driving surround speakers? 

What volume are you setting on the phone’s headphone jack to test this with?

Are you running S1 or S2? 

Is this on your Era 300?

As above is it set as a surround? (Input option is disabled when configured as a surround)

If not, double-check that the plug is fully seated in the socket.