When I turn on my TV with HDMI input, as often as not I have an HD picture but NO Sound. I always have sound with antenna input. An easy temporary fix is to restart the Roku, but the fix will be good for only a few days at best. Roku Customer Service has been little help. At first I chatted with someone, but I have not heard from them for the last 4 days.
For the first year I had the Roku, I had no problem. I have had the sound problem for about a month or so. The Roku's software was automatically updated Oct 2, 2018. It appears to me that the software update had bugs.