http://pspoachers.tk/index/extra_in_game_music/0-20Follow the link above directions on how to install is below :
How to install:
After you have downloaded the file you will want to extract the file. Then open it up and go into it's "seplugins" folder (if you don't have a game.txt and vsh.txt you can just copy the whole "seplugins" and paste it at the root of your memory stick and if it asks "Do you want to merge, etc, "seplugins folder press yes.). For the people that have vsh.txt or game.txt you will want to copy the music.prx and music_conf.txt to your PSP's "seplugins" folder. Once you have done that you will need to put "ms0:/seplugins/music.prx" into both your PSP's "game.txt" and "vsh.txt". After all that you are finally finished. Read below on how to use it.
Usage of Music.PRX: (Action = Button(s))
MUSIC VOLUME UP = SELECT|UP
MUSIC VOLUME DOWN = SELECT|DOWN
START/STOP = SELECT|LTRIGGER
PREVIOUS SONG = SELECT|LEFT
NEXT SONG = SELECT|RIGHT
MODE TOGGLE = SELECT|RTRIGGER
CPU NEXT = SELECT|TRIANGLE
CPU PREV = SELECT|CROSS
RELOAD = SELECT|CIRCLE
DISPLAY = SELECT|SQUARE
IN GAME MUTE = SELECT|HOME
LOOP = SELECT|START