Light MP3 worked on 5.50 GEN for me. I had it in the GAME150 folder, as, for some odd reason, it would crash on 5.00 M33 if it wasn't in there.
lol. for irshell, there is two volume bars. there is the psp's master volume, controlled by the volume buttons, and theres the irshell mp3 volume, controlled by the little volume controls next to the mp3 player controls. it should get pretty loud. the light mp3 volume boost wasn't that great anyway, it just ends up distorting the music... >.>