Do not download from random pop-up sites. The official source for legacy codecs is often XDA Developers or the GitHub mirror for MX Codecs. Look for a file named exactly: neon_1.49.0_armv7.zip
The solution often lies in a small but mighty file: the . 1.49.0 armv7 neon codec for mx player
After installation, restart MX Player. Go back to . Under "Custom codec," you should now see: 1.49.0 ARMv7 NEON . Do not download from random pop-up sites