I'm having this same problem. Its with the LG TV, not the ROKU. I know this because not only is the audio from the TV speakers dead but also the ARC port to a sound bar and the digital fiber output port is dead when on ROKU input. When I switch back to Cable, you can see the red lights in the fiber optic come on.
The first time I switched inputs to the ROKU port, a warning came up about sound but it didn't last long enough to really read it. It never came on again.
Thanks for posting here in the Roku Community, @Munchiniwa!
We appreciate you reaching out regarding this matter. We'd be glad to find you the best resolution possible.
Firstly, we need to consider the compatibility of your Roku player with a soundbar. Have you tried plugging in your Roku player to a different TV to see if the issue still persists?
Under this circumstance, could you let us know which Roku streaming player you are having this issue with? In addition, please specify the model of your soundbar for us to assess the situation better and provide you with an accurate resolution.
We'll be looking forward to your response!
Best regards,
Carly