Instructions
Check the volume control to confirm the volume is set to maximum. Press the "+" button on the left side of the PSP logo, below the screen.
Press the volume settings button if the maximum volume still isn't high enough. It is the second one to the right of the PSP logo, below the screen.
Access the AVLS feature. This stands for the Automatic Volume Limiter System, and it limits the maximum volume of the earphones to minimize outside noise. You will see it immediately after pressing the settings on. Set AVLS to "off." The volume on your PSP should now be louder.