Okay, so I really want to record my electric guitar on my iMac. I have a usb/audio cable. However, when I plug it into the computer while GarageBand is running, the sound on GarageBand completely disappears. Everything can play and the computer still picks up the guitar's sound, but there is no sound, just the visuals of garageband. When I unplug the guitar and restart the application, sound works again, but If I plug the guitar in, I come back to the same problem. So, how can I fix this? I tripled checked the volume levels. What is wrong?