Key click volume iPhone 4

Hi,
I need to boost the volume of my key click sounds (Im using an iPhone 4 as part of a psychological experiment involving acoustic feedback).
Its been no problem to replace the default key click sound (Tock.caf) with my own sounds (e.g. by using Cydia and Winterboard in order to quickly load customized sound themes). However, the max volume for key clicks has been quite low since iOS 3.1: As soon as my sound is played back as a key click sound, it gets attenuated. If I play the same sound file over the built-in iPod, its much louder. Amplifying the volume of the sound file before uploading the file to the iPhone does not seem to change anything.
Does anybody know where in the iPhone file structure the attenuation takes place? There has to be a file I can edit in order to disable the attenuation...
Thanks and cheers,
Klemens
iPhone 4 iOS 4