USB audio in iMovie question

Hi all!

I have an imac I bought new in Feb 2013. It has no audio input. I am trying to make a video in iMovie. I need to also have a music track while I am playing guitar to go with this video. I can't figure out how to do that.I am playing guitar through a POD, out through the usb, and into the imac via usb. That works well. I can't figure out how to get a backing track with it. I don't think I can use the mac's audio, as I have it set the the Pod as the audio source. Thanks! Phil

BTW, I have tried using the mac's built-in microphone, but it does something strange. Everything sounds ok when I start playing, but after 20-30 seconds it starts to go into this strange 'phasing' kind of sound and makes the music very hard to hear. So, I guess I have another question; Just out of curiousity, why does that happen? I am not playing loud at all.