via http://www.emucr.com/2011/08/mp3playerplugin-v23.html

MP3PlayerPlugin v2.3 is released. MP3PlayerPlugin is a plugin for PSP. It's a nifty app that allows you to listen to your favorite tracks while on game mode. The plugin is compatible with CFW 5.00 M33, 5.03HEN, 5.50GEN, 6.20TN und 6.35 PRO.

MP3PlayerPlugin v2.3 Changlog:
- The menu draws from (DRAW) deleted items
- Some bug fix
- Code Optimization

Download: Here