For those who find the PSP's volume too soft, I happened to stumble across this plugin.
Just stick in ms0:/seplugins and add the appropriate line to ms0:/seplugins/*.txt

It increases the volume a fair bit.  Am not sure whether this somehow increases the maximum volume capability, or applies a "dumb gain" - if the latter, you may get some sound distortion (though it may be preferable to inaudible sound).

Source says to press Home+R, but it was enabled for me without that key combo.

- Source: [QJ]