Same issues here - Roku support tries to deflect. Ill be watching a show and audio just drops out. Only fix is advanced system settings-reset network or unplugging and plugging unit back in. Just started doing this 3 weeks ago. I've tried running Roku thru different HDMI ports and cables with no difference. What's strange is Roku menu clicks and app loading screens push sound thru, just not any volume from shows in any app. Volume did also start cutting in and out like small buffer pauses when it is working. My guess is a handshake issue with new firmware they just pushed out a month or so ago.
Assuming your Ultra is a 4800, when you lose app audio but have UI audio, try switching Settings/Audio/Digital output format to Auto for 3-5 seconds and then back to Passthrough - it should reset the audio stack and any latency/dropout issues should be fixed (until the next time), and saves the restart.
(If you didnt have your 4800 in Passthrough mode to begin with, you should, because Auto can cause all the issues you describe due to it transcoding/converting all non-DTS audio to the highest detected Dolby level for HDMI output)
Otherwise, an unstable network connection (packet loss/latency/jitter) or a bad/inferior HDMI cable can cause the "cutting in and out" you describe, so you'd want to troubleshoot both of those.