One of the things that makes Siri and Google and Amazon effective is that they literally have hundreds of millions (nay, billions) of devices using their voice assistants, so they have the numbers necessary to improve their services. With such a relatively low volume of devices, I think it will be hard for Sonos to compete on quality.
Also, it seems like a weird thing from a financial and marketing standpoint. Unless it is demonstrably better, somehow, than Alexa or Google or Siri, it’s not gonna get anyone to buy a Sonos over another speaker. And to make it really good they will have to spend a ton of money and development. So it just kind of seems like one of those vanity projects that maybe might not have the best strategic underpinning.
Unless, somehow, it is noticeably better than the other voice assistance. If they could pull that off, it would be interesting. Anything shy of that seems like kind of a weird endeavor.