Ok so I could not find anything about the loss of volume or sound. I have a Roku ultra and about 3 to 5 times a day. I have to reboot the Roku to get my sound back. It usually happens when I pause the video and then when I come back and restart it, there is no sound rebooting. It always fixes it but it’s quite annoying to have to do it so I frequently. #sound
If you have the newer Roku Ultra (4800/4801/4802) models, try disabling the Auto Power Savings feature and see if that resolves the issue for you. Has reportedly caused some HDMI issues. (loss of communication).
Settings/System/Power/Auto Power Savings/20-min [Disable]