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Could be the TYPE of audio being delivered. Is your Roku set for DOLBY Digital output instead of PCM output? It should be PCM stereo, because the OLDER TVs didn't do Dolby transfers...only PCM transfers. Don't look at your TV's audio, look at the Roku's audio settings and any other HDMI connected device's audio menu.
The TV's job is simply to transfer the incoming HDMI PCM audio to the TV's optical output. And your TV speakers should be turned off too. But you know that already. The LN series of TV have been around for about 8 years, so you know your TV quite well.
But that's the one thing a lot of customers miss...PCM vs Dolby AC3