How To Separate Audio From Windows Movie Maker?

I want to be able to take the audio, or clips of it (so that it's mp3?), and edit it in audacity. I'm trying to deepen someone's voice. And while I'm at it, I'd like to know how to re-insert that edited audio back into its corresponding clip as if it were the person's own voice (without the original voice in the background). Thank you very much.