This post is reporting a problem as well as my solution.
I was trying to play the Roku branded series UFO Cowboys. I got the intro and background music, Closed Captions, but no narration. I could tell there was narration and speech, but had no audio. Deleted the channel and re-added it. Nope. Turned off closed captions. Nope. Reset Roku device and reconnected HDMI. Nothing. Tried a different TV and Roku. (Living room - was in Kitchen before) SAME ISSUE! A different series was fine. Hmmm
What I tried that finally fixed it was to go into settings for the series and configure Spanish as that audio track. Poof! I heard Spanish voices! I then repeated the process and selected English. poof! All fixed!