Not sure if its related...we'll see...but I had no sound issues until today...sometime after I changed the sound to 'amplified'. It was on normal for a few weeks since I got the Roku ultra with no problems.
Earlier today I changed the setting because its louder and a while later the sound started going out randomly. It'll come back on 5-20 minutes later if not messed with, though...not sure what that means.
If you are still unable to resolve the issue, can you please provide the serial number of your Roku device from Settings>System>About? I'll be able to pass it along to our Support team who can assist you further.