Roku remotes only control tv volume. And its upto the tv to control the audio device volume by hdmi-cec, but in this case won't since you're using the optical cable. In that case an aftermarket universal remote that control roku and soundbars is needed.
As for why its not controlling the samsung there have been many others with the same trouble with samsung and sony tv's and roku remotes. Unless a software upgrade fixes it, I'll would go with universal remote.
Edmund
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