How are your Advent speakers connected to your system?

Via wiring from the tuner/amplifier to the speakers. 

You don’t say how the audio signal gets to your Sonos gear, but lets assume its via a Connect.

Feed the audio from the Connect into your amp, and group Sonos speakers with your Connect they will be in sync. Sounds like you migh tbe taking the audio from your receiver and that to the Connect, and that will never be in sync with speakers connected to the receiver.

A few amplifiers have the capability to send their input signal to the record out (or similar) and take the record in (or similar) signal, amplify it and send to the amps speaker output.

In this scenario you can use the amplifier input as the base audio source and all your speakers will be in sync because the Connect effectively becomes the audio source, as described by @controlav 

I suspect this is an AVR not a HiFi amplifier, and that you are using a Connect or Port to feed the AVR?

If that is the scenario, then the Sonos units are synching perfectly, but audio processing in the AVR, over which Sonos has no control, causes the lag.  Many AVRs have a ‘direct’ mode (it may be called something else) which eliminates the sound processing and allows perfect sync.

Hi @Carlab 

@Ralpfocus is spot on here - if your source of music is not from Sonos, the only way your 3rd-party speakers will be in sync with Sonos is if they are set to “monitor”. Typically, you would have Tape In (play) and Tape Out (record). Flicking a switch labelled “Tape Monitor” would allow the selected source (let’s say CD) to be fed to the Tape Out (Sonos Connect/Port) then back from Sonos to Tape In and played over the HiFi speakers.

Sonos introduces a delay as it digitises the audio and prepares it for sharing across other Sonos rooms. If the source is Sonos, all this is done before your HiFi gets the audio, so there is no delay and Sonos and your HiFi play together. This presumes that your HiFi is analogue in nature.

John is absolutely correct: the sound processing in some home theater systems causes a delay.  I often listen to spoken word, saved as mp3s on my NAS, and play them throughout the house as I clean or do other chores that have me walk from room to room. Most of my Sonos are speakers, but I have a Port connected to a home theater receiver, and a Connect to a normal amplifier.  The home theater speakers and my kitchen speakers can be heard simultaneously, and there is definitely an echo effect, kind of like hearing a public speaker in a large empty and otherwise quiet room.  As soon as I switch the home theater to Pure Direct mode, it’s back in sync perfectly.

So there are two possible places delay is being introduced: the input delay on the Connect, if that’s how the signal is split to both Sonos and your traditional setup, and any audio processing on the receiver driving those speakers.  Feeding the signal first into the Connect/Port, then into the amplifier, and turning off signal processing on the receiver should correct for both.