Diagnosis
The line-in input is not activated in the app, or you are using an incompatible cable or source.
How to Fix It
Activate the line-in source. Open the Sonos app. Select the "Browse" tab at the bottom. Under "Inputs," select "Line-In." You should now hear audio. You can also set it to auto-play in the settings.
If That Didn't Work
Check your cable and preamp. The Era 100 requires a line-level signal. Most turntables need a separate phono preamp. Connect your turntable to a phono preamp first, then use a standard 3.5mm to 3.5mm cable from the preamp's output to the Era 100's line-in.
Heads Up
The Era 100 line-in is a 3.5mm analog jack. It does not provide phantom power for devices like condenser mics. It is for audio output only.