Does this cutting out of your home audio only occur when using your Roku Ultra or does it happen when you use other device, or play audio directly from your TV? Have you tried using another HDMI cable used to connect your soundbar as well as using another HDMI port on your TV?
It's possible that there is an issue with your soundbar that you may need to contact Vizio to assist you further with.
To confirm my issue, I was watching Netflix last night through the Roku ultra. Every single time in episode has changed I had to unplug and replug in to Soundbar just to get the sound to work. I then disable power to the Roku ultra, and watch Netflix through the apps on the TV itself. The sound never went haywire.
If you are not experiencing any playback issues when connecting the Roku device directly to the TV, then this is mostly likely an issue with your connection between the TV and the soundbar. Have you already tried contacting the soundbar manufacturer regarding the issue you are experiencing to see what insight they can provide?
I can verify it is not an issue with the sound bar. I am having the same problem as OP. I recently upgraded to the Roku Ultra from a Roku Streaming Stick because the Stick was overheating regularly and I have had nothing but trouble with Vizio soundbar compatibility. It started with the sound mix being completely off: music and sfx played normally but dialog was inaudible. I did a complete reset of the sound bar which helped for the most part but now I have the same issue as OP. Every time a new episode starts the sound cuts out completely. I can verify that this is not a sound bar issue because it only happens when streaming through the Ultra, it does not happen with streaming through the TV apps. Given that this seems to be a common issue, I am really regretting getting the Ultra and will likely return it.