I just started having this issue too, on my Roku 2. I lost the sound on both Netflix and Prime video. I found a workaround is to select a different audio option within each program I stream, but that's a pain. It works if I select one of the different "English" audio options, such as adding the enhanced audio that announces what the onscreen characters are doing. Notably, it is only doing it within some of the Roku apps, (the menu sounds are fine, which validates the rest of my audio chain is fine,) and selecting the alternative audio helps, and it's on multiple apps.