You’d need to adjust something on that external soundbar. The CONNECT and the Sonos One will by design be in sync, so it’s somewhere outside of Sonos where you should be paying attention.

And that being said, there’s an extremely remote chance that there is something wrong in your Sonos devices, which could be reset by simply unplugging them from power for 30 seconds each. That would force a reload of the software, and a reset of any potential hardware issue there might be. 

When you say wired to the sound bar, do you mean the output from the Connect is to the input on the sound bar, or the sound bar has an output that is connected to the input on the Connect? What audio source are you using?

Wired meaning output from Connect, input into Soundbar. Playing audio via the Sonos app. Have tried using various music services (Spotify, Soundcloud) and problem is consistent. 

I still am of the opinion the delay is caused by the input in to that Soundbar, and not the output from the CONNECT. Try hooking up some headphones, if you have the appropriate cable, to see if the CONNECT’s output and the Sonos One are in sync. As long as they are “grouped” together, they absolutely should be, and the issue is with the input on your Soundbar. 

Restarting the system didn’t work.

My RCA cable is 12 feet long. Do those cables cause delays? 

Well, technically, yes, electricity has a finite speed in a material,  but you’d never be able to hear that delay. 12 feet would not be an issue.

Again, I’m suspecting the delay is in the soundbar, not outside it. Did you try the test I suggested, with the headphones?

Tried that and it doesn’t seem to work… 

My old sound system used Amazon Alexa units and when you had multi-speaker groups there was a way to manually adjust for sync of audio. There is no equivalent feature on Sonos? 

How else can I solve this as the system is basically useless when used as a group right now. 

I think your best bet at this point is to submit a system diagnostic , and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.