An audio-description track may be selected; it is an accessibility soundtrack rather than a subtitle or buffering fault.
Why this happens
An audio-description track adds spoken information about actions, scene changes and other visual details. It can sound like an extra narrator between the ordinary lines of dialogue. That is different from an accidental voice-over caused by a second app, and it is also different from captions displayed on the screen. Some sources provide several soundtracks with similar language labels, so the selected English track, for example, may need a closer look.
Open the audio-track control for the programme currently playing and read any description or accessibility label beside the language. If another normal soundtrack is available, select it and confirm the extra narration has stopped. The choice may be remembered by the player or by an accessibility preference on the device, so note whether it returns with every new programme or only with one source. Do not disable accessibility settings blindly across a shared household device; first establish which track or preference produced the result you are hearing.
A useful way to work through it
Read the current audio-track labels, including any description or accessibility note.
Compare another available soundtrack on the same programme.
If narration keeps returning, review the documented audio-description preference for that device.