SOLUTION: An old optical cable from my Bose Solo solved the problem. I'm unable to get the Roku supplied optical cable to securely fit into the ports on the TV and Soundbar.
I just received and installed a Roku Smart Soundbar. It is connected to a Samsung TV without ARC-capable output, and, yes I have also installed the optical cable. The sound works fine on Roku streaming channels and when connect to a Bluetooth device for music. When I watch cable TV and switch the TV output to external speaker, no audio comes out through the Roku soundbar. I've plugged and unplugged the HDMI, optical, and power cables multiple times and still nothing. What's going on here?