I have the same issue but my Roku is working on other tvs and even on my tv when not plugged into the sound system. But if I don't plug it into the sound system, the sound does not work. It was working in the past and then just stopped working. My tv sound system is otherwise working.
My roku is a streaming stick- SNC237AE652631
MY TV is LG 60PS60
My sound system is Sony DA2400ES
Thanks for your help.