Sony TV won't use it's own speakers when Arc Ultra is connected

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?