This usually means the NEON codec version does not match the MX Player app version. Update both the app and the codec pack simultaneously.
When MX Player lacks the licensing or built-in support for specific premium audio formats (like DTS, DTS-HD, AC3, or EAC3), this custom codec bridges the gap. It allows the media player to utilize your phone's hardware to decode high-definition audio and video smoothly. Why Do You Need a Custom Codec? Mx Player Armv8 Neon Codec