I have a Roku 4K connected to a Vizio M658-G1 (new TV). I can see video but no audio. My set is is Roku 4K to TV, TV optical audio out to Klipsch Sound bar (Sb120). Current TV setting is Digital Audio out via PCM, and Roku HDMI and S/PDIF set you PCM.
note with my Apple TV 4K I can have TV and Apple TV set to Dolby digital out.