Since you get sound when you connect the Roku directly to the soundbar then the problem is the TV.
Check the TV audio menu to make sure that the audio output or ARC is turned on and set to the correct audio type for the soundbar.
If that doesn't help then the TV is broken. You can bypass the TV by sending the audio directly to the soundbar. You may need an HDMI input selector that is ARC compatible or one that extracts the audio. This won't help with the built in TV tuner.