Sounds like you’re not interested in the way CEC works, and the way the Sonos Arc Ultra is designed to work. You might try turning off CEC, which Sony may call Bravia-Link, so that the Arc Ultra isn’t automatically recognized by your TV as a speaker. If that works, you should be able to use the TV’s internal speakers. Your other option is to remove the HDMI cable each time. 

Disable CEC (called BRAVIA Sync on Sony) when you do not wish to use the Arc Ultra.  Otherwise, CEC recognizes the soundbar and defaults to it, because it (like I and many others) doesn’t have a clue why anyone in their right mind would prefer the tiny, tinny TV speakers over a good soundbar. 

I have tried everything even plug the HDMI cable on the splitter not connected to ARC but it still change the sound output to external speacker instead of TV.

it was not happening on other soundbar. It is happening only on Sonos

yesterday I called technical support and no solution from them either

Did your other soundbar support and connect by HDMI, HDMI-ARC, or HDMI-eARC? 

yes. It’s Samsung Q990C and I upgraded to Sonos and I’m disappointed with this 

Which connection did it use?

HDMI-eARC

also there are no option to turn off ARC too

And have you disabled CEC? 

yes

are you from Sonos technical team?

No. You’ve posted on a User Community forum. Why do you ask?

I need help ASAP

Then you’d best call Sonos Support directly to discuss it. Posting in this community forum doesn’t put you in touch with Sonos Support folks. 

The terms you agreed to makes it very clear that this is a user community. 
 

Good luck getting sorted: I’m out. 

I ask again, why would anyone prefer the crappy speakers on the TV to a quality soundbar?  I’ve had surround sound of all kinds connected to my TVs for decades, and not once in all that time have I sat and thought “Gee, this would sound so much better if I were listening to the TV speakers!” 

Which is exactly why CEC is designed to default to the best audio it can detect. 

Install a CEC blocker between TV and ARC. You’ll then lose some convenience features, but these are not as important to you as speaker selection.

what features?

CEC also carries volume information between the TV and Sonos. Here is what CEC contains, per Wikipedia . 

Exactly what kind of Sony TV do you have?  I’m experiencing the same problem on my Samsung QN90A.  And I found posts with people having the same problem with their LG TV’s as well. 

It is unbelievable that Sonos cannot be bothered to fix this problem, whatever the solution requires - either fix their implementation of the protocol or provide a workaround (perhaps a configuration options) for the way the CEC is implemented on some??! or all the TV’s.   Why should I disable CEC and sacrifice the features it provides, such as volume control?  

Why can Sennheiser and Samsung do it and Sonos cannot?  I want to be able to route the sound to the whatever device I would like to, from the TV menu, as intended, be it Sonos, internal speakers or the headphones connected to TV.

I have the same issue. A call to support blamed the Sony TV. I think it’s more like the Sonos implementation of CEC as other soundbars don’t have the issue. I’ll add a cheap Visio to the list of soundbars that don’t have an issue switching to internal speakers. I would expect better from Sonos after paying top dollar for it.

i

Check the specifications for CEC. Last I looked, there was only one audio ‘sink’ allowed. Other soundbars may have gotten around this by not using CEC, and/or connecting via optical, which of course has no CEC by definition. 

First let me say that I don’t understand the reason why this thread was started:

The purpose of a soundbar is to negate using the tiny internal speakers of a TV for better audio.

When attaching a soundbar you go into the TV’s sound/audio menu and tell it to use: *

• Optical Out
• HDMI ARC Out
• HDMI eARC Out

The latter will also require enabling the TV’s flavor of CEC; as there is no standardized implementation codec. If you want to switch back to internal speakers, you’ll have to go into the sound/audio menu and select them.

Once the soundbar is installed I see no reason to switch back to internal speakers. The problem for me would be the TV auto-switching back to internal speakers. 

BTW, Sonos doesn’t implement CEC, that’s a function of the TV.  It varies by manufacturer as stated earlier.

Here’s a brief explanation of CEC implementation. Read the full article at this link.

• How Can I Tell If My TV Has HDMI-CEC?

• The majority of televisions manufactured today support HDMI-CEC, as do the majority of devices you might attach to your TV, like media receivers, soundbars, Blu-ray players, streaming devices, game consoles, and more.
• Despite the near ubiquity of HDMI-CEC at this point, you often won't see "HDMI-CEC" printed on a specifications list. Although HDMI-CEC has been part of the HDMI standard for years and has a perfectly fine name, nearly every manufacturer insists on using a branded "trade name" for HDMI-CEC instead. This is very similar to the way manufacturers have implemented Miracast, but given it a special name instead.
• It's pretty annoying, and they only do it because it pushes consumers toward purchasing items that appear to work together because of shared branding. For instance, if you're shopping for a new TV and you see a Sony TV labeled as "Bravia Sync compatible" and a Blu-ray player also labeled as "Bravia Sync compatible," you might be inclined to buy those two devices thinking they work together using some specialty Sony feature — when in reality that's just Sony's in-house name for HDMI-CEC.

In conclusion maybe I’m missing something in these complaints. I’m really trying to determine their legitimacy as a Sonos issue.

 * Be very surprised if anyone is still using a TV with RCA audio-out-to a compatible RCA-input soundbar. Provided still exists.