A fundamental element with Sonos is a delay when play is started, whether from Bluetooth, line-in or streaming. It happens even with a single speaker in a one-room household. It’s needed ready for synchronised multi-room play and there’s no workaround. Consequently your wish will, I fear, remain unfulfilled. 

Too many questions for a simple answer.

Essentially, no, you can’t do what you want (play in sync). All Sonos analog line ins, such as are the Sonos Era 300, are subject to a minimum of a 75ms delay, so the music can be buffered to play across (potential) grouped rooms. This makes a Sonos speaker by itself not appropriate to sync with other speakers.

You could go the long way around this by feeding your signal to a Sonos Port, and then feeding that Sonos Port’s signal to your amplifier via a tape loop or equivalent, and then the amplifier’s speakers and the Sonos speaker would be in sync. 

So in your work-around the Port would delay the signal to the amp, and that delay would be equivalent to the delay inherent in the Sonos speakers? Thus the Port simply serves the purpose of producing a signal delay. Am I understanding you? Thanks.

Essentially, yes. The data enters the Sonos port at time X, it gets delayed per Sonos’ software to X plus 75ms, and then exits to your amplifier , to play essentially in sync on both the amplifier’s speakers and the grouped Sonos speakers. But that means anything you want to play on both speakers must go through the Sonos Port first, before you can play it in sync. 

Think of the Port as a cassette deck, if that helps. 

All Sonos analog inputs have this. Even when you stream music from the Internet, it is delayed by that 75ms, you just don’t notice as you have no other playback source to compare it against. And it’s been part of the software from the beginning, and can’t be modified/reduced, as it’s what makes Sonos able to stream music in sync across Y rooms at the same time.