Removing audio... wrong approach?

I'm attempting to create DVDs of old family video tapes, and sprucing them up in the process. I've digitized the VHS tapes just fine into iMovie HD 6 as DV, but along with it came the empty audio track. This means my movie has a faint hiss as its only sound, which I'd like to remove and replace with music.

So I tried using Extract Audio to break out the sound tracks on the movie, with a goal of deleting them once they were broken out. But it seems to take an excruciatingly long time, even on my fast Mac Book Pro, to Extract Audio. A 10 second clip takes about 10 minutes, approximately. At this rate, it will take all day to just extract the audio from my hour-long movie.

Am I doing the wrong thing? Is there a better way to remove the built-in hiss and replace it with my music tracks?

Thanks in advance!

Macbook Pro Mac OS X (10.4.9)