My Ruku Express is having the same issue. I have tried several different apps and they all seem to have the issue where the sound cuts off every so often. I have restarted the device several times but it continues to do it. One trick that gets it working without me getting out of bed is by going on to the Roku app, turning on private listening mode and then turning it off again. The sound comes right back.
I don't know that your issue is the same as mine, but here's what I did to make mine work.
My system - Roku stick on a Vizio TV (no external speakers or soundbar).
On the Roku stick menu, go to Settings -> Audio -> HDMI -> and set to PCM - Stereo. It appears that the audio HDMI auto detect routine has a defect in it where it fails to switch over after one or more of the states was selected. The Audio Mode setting did not make a difference. This means no Dolby output, but at least I have sound.
Hope it works for you while Roku find the bug and squashes it.