Try changing decoder setting (e.g. Dolby) on your sound device. If it has a “direct” setting I would try that first. I think Roku has an incompatibility bug with some decoder (s) as this happened to me too.
Thanks. My soundbar does not have a "direct" setting not to mention I've tried all possible settings anyway.
You hit the nail on the head with, "I think Roku has an incompatibility bug...". Roku absolutely has incompatibility bugs!
I ended up plugging the Roku box into a remote controlled electrical outlet I had on hand and when the sound goes out, I cycle that outlet off/on and proper sound always returns in an instant. What a convuleted workaround.