I am just reading this string of comments realizing that it's nearly one year later. I have been having exactly the same issue with my Roku Ultra. I did not have it at all with an earlier Roku Ultra model. Any suggested fixes would be appreciated.
Personally I don't think it has anything to do with the Roku. I think it's a bug in the LG firmware. Mine has done it on occasion, even if I'm not using the Ultra at all. I can be watching TV or my Shield, and I realize the TV speakers are in use, even though my AVR is on. In my opinion, that's something the TV is causing, not any external device.
