Make sure you have iClarified added as a cydia source. You can find directions on how to do that here.
Press to launch Cydia from your Springboard.
Press to select the Sections tab at the bottom of the screen.
Press to select iClarified from the list of Sections.
Press to select Volume Boost (2.0.1) from the list of Packages.
Press the Install button at the top right hand corner of the screen.
Press the Confirm button at the top right of the screen.
Once installation has completed press the Return to Cydia button at the bottom of the screen.
You will be returned to the Package details page. Hold down the Sleep/Wake button for 3 seconds to bring up the Power slider. Slide it to the right to turn off your iPhone. Then push the Sleep/Wake button again to turn back on the phone!
You will now notice an increase in the volume of your phone :)