Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
While I can't offer anything specific, the Roku forum admins will want to know if your Ultra is connected directly to a TV, if it's connected to a soundbar (directly or to the TV first), connected to an AVR, and what your audio options are set to. Also, is it just this one series (that I am assuming you are watching on Prime Video), or is it occurring with other Prime videos or any other channels?
Roku Community Streaming Expert
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