Mine is model: 4660x2 and is roku ultra. If I unplug and then plug back in, it has been fixing. It's done 3x this morning already. It's when I switch from one app to another (like Hulu to Youtube) and even if I go back to first app - no sound, TV still plays the sounds as i choose buttons and move on menu.
If you are unable to resolve the issue, can you please provide us the following information: -Roku device model -serial number -device ID -software OS/version (these can all be found in Settings > System > About) -does this issue occur on a specific channel? if so, what channel and what version/build is the channel (this can be found be selecting the channel on the Home screen and pressing the * button) -tracker ID when this issue occurs (when you see this issue occur, press the Home button 5 times, followed by the Back button 5 times)
Once we have this information we will be able to pass it along to the appropriate team to investigate further.