It will and is needed where line in is needed for speakers not blessed with these.

Yes. That’s how I use mine. CD-player connected to the connect, group rooms with connect or make one Sonos device the preferred speaker for it.

Thanks. So on the app, how do I do this?

System-Name Connect-“Automatic playing” (my system is set to Dutch, so this is my translation).

I've got it, but the volume level is lower than the volume level from streaming?  

Change the line level settings to get the volume levels up. Even 10 is ok, the max.

What are you connecting to the Line In sockets on the connect? If it is a turntable and you have the line level setting to 10 and it is still a lot lower volume than streaming, you may need a pre-amp for the turntable. Some turntables only have a Phono level output which is lower than a Line Out signal.

My settings for Line in:

• Volume limit: off
• Source volume: Level 2 (A/V-component).

My settings for Line out:

• Line out level: Fixed. 

I’d expect that @lohr500 has it analyzed correctly, and you need a pre-amp between your turntable and the Sonos line in.