Hi @KevinH1996 

Welcome to the Sonos Community!

There shouldn’t be a problem, but if you do so and after a time you hear the volume go quieter by itself, disconnect the 3rd speaker, reboot the Connect:Amp, and don’t try it again (the volume ramps down to protect the digital amplifier from overload, so no harm done, but don’t persist).

To connect, just wire the 3rd speaker to either the left or right channel, in addition to the one that’s already wired (In parallel, as in connect each wire from the speaker to a terminal just like the existing connection).

You may have spotted a flaw with this - you must choose the Left or Right channel for the new room, and cannot get the other.

If the Connect:Amp were a newer Sonos Amp, you could use Dual Mono mode so that all 3 speakers play both left and right, regardless of which terminals they’re wired to. You would lose stereo imaging in the existing room should you do this.

My advice is to either upgrade to an Amp (you also get AirPlay support with the Amp, not to mention more than double the power output), find out where you put that 4th speaker, or where to get an individual replacement for it if it’s faulty. eBay would be the first place to look.

If your existing two speakers are in different rooms, then presumably you don’t care about stereo, so just wire that speaker up and try it!

Hey man thanks for this info this is great. I actually do have the new amp! So what I plan on doing is wiring the third speaker on any of the existing channels and change it to dual mono in the Sonos Amp, correct? thanks again! 

Exactly - then all 3 speakers will each play both Left and Right channels. Enjoy!