There is an API already published by Sonos. All you need to do is get Google to stop their frivolous lawsuits, and implement that API in the YouTube client. 

That is best for Sonos to sought out with Google. If google assistant works then I believe we should be able to support the google cast. 

Let me reiterate. Sonos has made the API available, several companies have integrated it into their operating systems. Sonos doesn’t have access to Google’s own OS code, and can’t force Google to implement it. 

Should Google do such a thing? I believe they should. If I was a paying customer of Google, I’d be complaining regularly to them. 

However, I’m also aware of the legal issues between the two companies, and the simple fact that Google didn’t implement their YouTube music integration as well as they had their old Google Play music integration is a fairly significant ‘tell’ in my book. So I have very little hope that the people in charge of further Google integration with Sonos are interested in increasing any commitment to the platform, and more likely trying to push their own hardware and software solutions in order to lock the user into the Google ecosystem. 

Let me put my understanding in, if Sonos supports the google cast or chromecast similar Airplay, the device gets automatically detected by youtube music. I am not able to understand why should an API be exposed and then be implemented by music streaming applications.

I do not think this is a way any protocol is implemented.

BTW, what is that API that you are pointing to ?

SONOS API.