Having the same problem. My Roku was bought June 2018. Started losing sound 3-4 months ago. The only way to get sound back is to reboot the Roku (turning the TV off and on does nothing, nor does changing inputs). System was last updated 12/7/20. I've been rebooting my Roku once a week for way too long.
All sound is gone. All channels, all inputs, basically if it goes through Roku it's silent movie time. Plex, Amazon, Youtube, Tubi, Roku, doesn't matter. Even the "bloop" when I press a key on the remote is gone.
Don't know if it's related but for the past few months my lip-sync is way off. Sometimes by several seconds. Again, all sources. Except the "bloop" syncs with when I hit a button on my remote.