I have a guitar to USB cable, I have the USB adapter for my iPad. I am trying to record my guitar in garageband, when the guitar to USB cable is plugged in I cannot hear anything, not even the metronome. If I try to playback what I recorded I cannot hear it, even with headphones. I unplug the cable and I can hear. I don't know what I am missing so that I can hear my guitar through the amp while I am recording.