I highly doubt this is possible without damaging the Arc, and you will definitely void the warranty on a pretty pricy piece of electronics.  As to Trueplay?  There's no way it would work, it expects sound from an Arc to actually come from an Arc.

Damage both physically and electronically.  

What do you want to achieve by this? If the objective is to get a homecinema set up into the Sonos universe, this can by done by a Port.

I’d like to get better Atmos response by placing a pair of in ceiling speakers closer to the listening area. Eliminate the Amp and wire to the in wall speakers I have now.

Even if you could wire in ceiling speakers to the internals of an Arc, your set up will probably not sound right because the Arc computes it's Atmos sound to make it sound like there are speakers above you - this sound stream will be different from a stream computed for speakers that are placed in a above you.

I have done something similar to what MML is considering. But I have only done it with a Sonos sub gen 3 that is paired with an Arc and two Play 1’s. Disassembled the sub added banana jacks to the bottom connected directly to the two opposing internal speakers. You can not tell I modified the sub until you turn it over and see the 4 banana jacks in the recess in the bottom of the sub. Did this same mod for a family relative with simple tools in his living room. Take the speaker level output from one speaker in the Sonos gen 3 sub and convert to line level with JL Audio Loc22. Tried using the more popular AudioControl LC2i but its frequency response only goes down to 33hz. Next take the line level from Loc22 and inject into a miniDSP2x4 HD. Used a calibrated microphone and Room EQ Wizard (REW) to setup 3 additional powered subs to the Sonos system. The miniDSP2x4 HD supports up to 4 subs. Sounds much better and I have good granular control over the subs with the miniDSP2x4 HD. There are quirks setting up and calibrating the system with REW but well worth the trouble for better sound, IMHO. I was considering disassembling the ARC to get to the Atmos speakers and use the similar components and techniques. 

There are significant difference between your hack and what MML is proposing.  You’re hack is only impacting the sub, and since lower frequency audio is omnidirectional, you aren’t having an impact on trueplay tuning.  You also are not looking to have amps drive speakers it was never designed for.  Lastly, your hack makes some financial sense as you are creating a system that is still essentially wireless, between the separate speaker/components anyway.

What MML is suggesting does none of that, and is essentially turning a wireless soundbar into a pretty crappy poor performing  atmos receiver that doesn’t really save you much money over an actual atmos receiver, particularly if you already have your passive speakers.  If you want your home theatre to be part of a Sonos system, that is what the Port was created for.

Danny

If I hijacked the topic post that was not my intent. Just trying to provide a possible option to readers. While you points are valid a from my understanding and having had a older Connect, a Port cannot be used for discrete low-frequency effects (LFE) or Atmos height? I think it can be used for the surrounds with the Arc? The Port could add some Sonos features to a typical receiver/speaker/component system which may be best for some setups. 

If one leaves the Sonos speakers unchanged/connected in the Arc and adds the additional line level converter, DSP, amp and speakers in the correct locations and tunes the DSP with REW in conjunction with Sonos Trueplay the system is enhanced significantly. This is was proven on the sub by listening and by REW graphs using a calibrated mic. This level of complexity is not for most users of Sonos. Just taking apart the high quality and engineered Sonos speakers without damage can be a challenge not to mention the follow on that comes after accessing the internal Sonos speaker components. That includes adding jacks, additional discrete components and hours of tuning the DSP and validating with REW.

The bass output was lacking in my ARC and single gen 3 Sub. The method discussed significantly improved it. 

The Atmos is lacking in my ARC. From listening tests with my Sonos system playing an audio file with discrete channel outputs I can tell the up firing Atmos speakers in the Arc are not angled correctly for my setup. I need to move about 3ft closer the Arc or angle the Arc. Moving the seats that close is a no go.  Angling the Arc moves the other speakers in the Arc off the correct axis. The thought/proposal is to add 2 additional height speakers similar to the subs. The new added height DSP could actually handle 4 speakers or 2 pair. The trick, similar to the sub, will be correctly timing the Atmos height sounds to arrive at he Main Listening Position at the same time. The DSP has provisions for the addition of delay to each speaker along with other typical DSP processing. 

I also moved away from higher end receivers to the Sonos system for several reasons. When many high end receivers had issues with HDMI chipsets and for the ease of use of a whole house system by family members. The family did not like the complexity of the receiver. The Sonos Arc with Sup and Surrounds is a sacrifice of some fidelity and tuneability for the features of Sonos eco system. My discussion is a method of getting some of the fidelity and tuneability back that most high end receivers provide. 

Hopefully Sonos will keep adding features as they have been and allow more advanced access into its Trueplay tuning setup. While I would like to have these advanced features, from my experience with family who own Sonos systems, many users struggle with proper basic speaker placement and the tuning features Sonos provides now.  Just getting those basic items correct as you and many contributors have stated will make the most significant improvement for most people using the Sonos for home theater purposes.

Alan