Sound Trouble With New iMacs?

I recently purchased a 2012 iMac To replace my 2007 model. The new machine has a 3.1 i7 processor. I made this purchase so I could improve the video and audio quality of instructional videos I create with iMovie. I was impressed with the video quality of the new onboard camera but I'm having horrible issues with the sound! Whether I use iMovie or GarageBand, anytime I try to record vocal and acoustic guitar or vocal and electric guitar with either the internal mic or an external mic, the results are awful. The recording sounds as if it's been processed through a flanger. As a result, I returned my iMac for a replacement as the techs at Apple could not help me solve the issue. Today I received the replacement only to find That the results were exactly the same. I have been asked to submit sound samples to the high level technicians at Apple. The sound output when using iTunes is just fine. The problem only occurs when I record. I love my new iMac but the sound issues are stopping me from doing my work. Does anyone have any suggestions as to how to correct this sound problem. I have heard that possibly using a USB preamp would help me bypass the sound processor in the Mac. Is this possible? Any ideas or suggestions would be appreciated.