My issue may be different than yours. My would work on MLB and some others, but when I went to Hulu or Disney then sound would stop - and it would not work again until I restarted my Roku stick.
I got this to work on 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 auto detect routine has a defect in it where it fails to switch over after one 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.