Volume Amplifier fixes the problem of quiet audio by directly manipulating the speaker audio stream and amplifying it by up to 200%.
The difference between Volume Amplifier and other Voice boosting tweaks out there on Cydia is that it does not alter a specific OS configuration file (which actually don't work), it actually manipulates the audio stream in such way that you will really notice the difference.
Jailbreak your iPhone using the iClarified Jailbreak Wizard if you haven't jailbroken your device already.
Tap to launch Cydia from your Springboard.
Press the Sections tab at the bottom of the screen.
Select Tweaks from the list of Sections.
Choose Volume Amplifier from the list of Tweaks.
Press the Purchase button at the top right of the screen.
Connect to your Facebook or Google account and pay the required payment of $1.99.
When returned to the package page, press the Install button at the top right of the screen.
Press the Confirm button to begin installation.
Once installation has completed successfully press the large Restart SpringBoard button.
Once your SpringBoard has restarted, tap the Phone icon and make a call. When on a call press the volume up button at the side of your device. A new screen will appear letting you know how much you've boosted the volume of the call.