if you're working solely with audio, you'll have no need for final cut.
it is possible to edit with quicktime, but it's rather finicky.
you need a dedicated audio editor. there are tons around (search this forum for "audio editor", you will get a lot of recommendations - also try versiontracker).
my personal favourite is sound studio, which came installed on my mac. you may already have it.
all you'll need to do is set up your imic correctly, make sure your mic levels aren't too high, and hit "record" in whatever audio editor you choose. then once the recording is done, hit "stop", save and edit the file, and then convert to mp3!
-Mark