Keep your downloaded libffmpeg.so file stored safely in a permanent directory outside of the app's internal folders. If an update resets your settings, simply repeat the step-by-step installation guide to link the file again. Conclusion
Once selected, the app will typically prompt you to restart. After restarting, your "unsupported audio" videos should now play with full sound. Troubleshooting Common Issues nplayer external codec
For "NPlayer external codec", a useful feature could be: Keep your downloaded libffmpeg
What is giving you trouble? (DTS, AC3, etc.) nplayer external codec
Look at the audio track information icon (often displayed as an "Audio" or "Tracks" menu).