Does it happen with every "channel" or show within "The Roku Channel"? Try the various options you can find under "Volume mode" when you press the * button on your remote while you're watching something. You might also want to experiment with the Audio options found in the main Roku Settings on the Home Screen (scroll up from "Home" on the left of the screen).
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.