Same thing happens with my new Ultra 4802x purchased from Best Buy and installed a few weeks ago.
Using it with a Bose TV Speaker sound bar plugged into HDMI ARC port 2 on my Samsung UN46D8000 TV (2011 vintage).
I just changed the Roku audio setting to Passthrough and will see if it becomes reliable.
My audio has been dropping out. Power reboot on the Ultra brings it back. It happens a few times a week.
Planning to test a new Apple TV system on another TV to compare.
I can still return the Roku for a full refund.
I also bought a Roku Streambar Pro for another TV (new Samsung) and it has not had this issue.
Have just left Comcast Xfinity service. We have 4 TVs and moving to T-Mobile Internet.
So far not a Roku fanboy.