Thanks for the follow up.
The Roku Wireless Speakers will not work with the Roku Ultra box, as it will only connect to either a Roku TV or the Roku Soundbar/Streambar.
The Roku Soundbar/Streambar can work with your Roku Ultra box, but it would effectively be redundant to be using the Roku Ultra as the Roku Soundbar/Streambar has a built in Roku OS system that you could use to stream.
Hope this provides some clarification.
Roku Community Moderator