you DON'T need to export the audio from the original clip
you CAN but don't NEED to
its kinda hard to see but you want to drag the clip into the timeline at the bottom as seen here
http://www.apple.com/ilife/imovie/ in the top image
notice right above the clip on the left there are 2 boxes ones blue ones not
click the one thats not blue and it will change the timeline view to look like the image in the bottom
you can now imort the new audio file and that is also were you will want to UNcheck the box on the far right thats next to the video file
by unchecking the box you mute the audio on the video file
if you export the audio from the video file you would then need to uncheck the middle box because thats were the audio will now be
hope it makes sence
EDIT
also once you have the new audio where you want it you can lock it there by selecting advanced tab and then lock audio at playhead (or something to that effact - again not on my mac right now so not 100% sure what it says)