Hmm.... I'm wondering if you could jack up the psp speaker's volume by increasing the voltage to the speaker inside?? (modding it..... soldering n' stuffs)
For CFW there is a plugin that allows you to boost speakers level.
There is also a homebrew called lightmp3 that allows you to boost the audio from within the app.
For hardware modding check this site out:
www.acidmods.com/forum/index.php/board,361.0.html
Plenty of cool mods there.