The Port is expecting a line level signal from any device. If you choose to push two turntables through a mixer, and feed the result from the mixer to Sonos, it wouldn’t know the difference.
Not quite sure why you wouldn’t just want to digitize the data, and use it without having the turntables and mixer involved, though.
And I’ll reiterate @jgatie s warning, for anyone else thinking Sonos is a great speaker for DJing….it isn’t. Any and all Sonos analog line ins are subject to a minimum of a 75 ms processing delay, so that Sonos can prep the signal to be distributed in sync across the worst case scenario of a 2.4Ghz wifi channel. This is built in to the software, and can not be “turned off”.