According to Plex's official support and the experience from the user community, the key to solving this problem doesn't lie in system settings but rather in the audio track selection within the Plex app. Follow these steps, and you should be able to resolve the issue:
Select the correct audio track while playing the video (the most direct method)
This is the preferred way to solve the problem. When you are playing a video with narration:
Bring up the playback menu: On the Roku remote, press the "Down" directional key or the "OK/Select" key. The playback progress bar and settings menu will appear at the bottom of the screen.
Locate the audio settings: In the playback menu, look for and select the "Select Audio Track" option or the "Settings" button, which is usually represented by a gear icon.
Switch the audio track: In the pop-up audio track list, you should see multiple options, such as "English (AC3 5.1)", "English (AAC 2.0)", and "English (Descriptive Audio)". Choose any option other than "Descriptive Audio". The video will buffer briefly, and then the narration will stop, restoring the normal audio.