I'm having the same issue on my ultra 4660, it happens whenever time the box isn't used for a day or so, it happens an all apps, (however a restart always fix that issue) but it's becoming very annoying as the box is just over a year old I bought it in november of 2018, this is my fifth roku box and I'm not happy with this one at this time...... as I said above A restart always fix this problem!!,
So, I've tried all the recommendations. PCM settings, reboots, etc... Sound has only gotten worse. I didn't feel like cracking it open (Potentially voiding warranty) to diagnose a potentially faulty thermal paste install situation, so I decided to engage Roku customer support via their website.
I just submitted it for a return/replacement request via Roku Customer Support website. That process says I should expect a call from Roku within 5 business days.
rabidsquirrel? are you using an AVR receiver/processor to process your HDMI cable audio/video from the Roku Ultra before going to the TV? (long story short my audio dropout/distortion problems went away completely after I disabled the audio in settings by setting it off and restart the box)
having said that even if you don't use the above-mentioned setup, give the audio setting a try-" set it to off" good luck hope it helps