I have a streambar pro connected to a samsung tv through the hdmi arc input. I have a cable box connected to a different hdmi input. I get sound when watching cable but if I switch to Roku streaming and then go back to the cable input There is no sound. I have to either restart the Roku or unplug it, get the sound back and plug it back in. I may have to just go back to a streaming stick and a soundbar that doesn't take over my tv.
We would recommend taking a look at the steps provided by our Support page here: Set up your Streambar to make sure the Roku Streambar is set up properly. It's important that the ARC and CEC are enabled on your TV.