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The Roku doesn't care at all what it sends the sound to. If the TV supports it, then it should pass through to the soundbar without an issue. The only uncertainly might be if your TV supports Dolby Digital Plus audio, since that what so many online streaming providers use now. If the TV doesn't support DD+, most likely you'll only get stereo sound from the soundbar, no surround (assuming the soundbar has that).
If your soundbar has HDMI inputs that allows you to connect the Roku directly to it, then odds are it will support DD+ as well.
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