iAmp is simply a software modification of the iPhone's firmware. It's in Installer like John said. The problem is many people have reported no improvement whatsoever after installing the app, so don't get your hopes up too high.
I had this same problem with my 1st iPhone. Hate to say it but the only thing that fixed it was taking it back. Of course it had other problems as well, like little bubbles forming under the glass, and a burnt/discolored area on the screen as well.New phone: volume has been great ever since.