Why won't my windows movie maker record a narration correctly?

I'm trying to do a recording thing from my I tunes. I do everything right. I have the start narration thing go then when my song ends i press stop. When i listen to it in the video its all weird it will repeat words, it will suddenly go soft then start loud, it will get all stat icy etc.I looked on you tube on how to do it and everything says I'm doing it right but i must be doing something wrong because this keeps occurring. I've tried doing different volumes to it but nothing seems to work. Please help me!!!!!! Thank you!!!!! :)