Re: Using a HDMI to composite converter. Video is fine but no audio.
If the device comes with a manual or any switches/options, you can try them. Since only the audio isn’t working, you can try adjusting the audio settings/formats in the Roku and see if any of them work better with the device. (Settings -> Audio.) If none of that works, well, that's why people here generally recommend buying converters from a place with a good return policy since they can be "hit or miss".