Look for an option to "External Codec" and point it to the downloaded file if it doesn't automatically detect it.
The process differs slightly between iOS and Android. Note that nPlayer does support external codecs on the Apple TV version. nplayer external codec
If the video plays without sound, check your Decoder settings. Switching from Hardware (HW) to Software (SW) decoding can sometimes force the app to use the external audio codec you installed. Look for an option to "External Codec" and
latest nPlayer external codec support - cpp-labs/ffmpeg - GitHub nplayer external codec