I have this issue too but it happens to me during the middle of streaming stuff. I can be watching a movie on Hulu or Netflix and the sound will go out. I have to restart the Roku to fix it. It happens almost everyday.
I wrote about this an hour ago when my Roku lost sound during playback of a streaming show. It happened AGAIN watching the same show. This happens on random streaming channels at random times and the only thing I can do is restart the Roku. I'm getting a bit fed up. I have 6 Roku devices so I'll swap this one out and see what happens. I haven't experienced this problem with other devices. The one with the problem is the newest one I bought. I've had Roku devices for about 6 years and this is the first big problem I've had. Hopefully it's just a bum device.