I have what I believe to be a similar (if not the same) issue. Audio will cut out for a few seconds, then come back. The happens about every 30 minutes or so. I have closed captions turned on so I don' miss anything. Yes, I have changed HDMI cables, tried different HDMI ports, result is the same. None of my other devices connected via HDMI have this issue with the audio, only the Roku.
Roku Express 4K+
Running the latest software for this device (since it pretty much auto checks each day)
11.0.0 build 419-CR
TV is a Panasonic TH-42PZ77U from 2008. For the heck of it, I did try a different TV. I connected the Roku using the same HDMI cable to my Vizio M321i-A2 from 2013. I am of the belief that when the Roku was connected to this TV, I didn't experience the audio cut out. I may have tested this for about 1 hour. So perhaps its some incompatablity between the Roku and my Panasonic from 2008? But I still can't understand why all the other HDMI devices (DVD player, DVR Spectrum cablebox, and any PC we connect to it) doesn't have these audio issues. Whatever Roku is doing, it needs to be a little more backwards compatible with older flat screen TV's then.