Sorry I'm on my PB not my iMac (trashed a bunch of the i-apps) but here are some notes I took from an Apple Camp:
when in iMovie, you can crop the audio track to coincide w/ the visual clips: drop a header/ marker where you want the splice, then go up to edit menu and select split video clip at playhead
you can also fade out the audio track (somehow a reddish/yellowish ballthe indicator for volume, w/ several points, simply drag down each point until it hits the bottom)
o go to view and select show clip volume levels
o a thin line will appear within the purple audio track
o drag down from that line
Sorry if they don't make any sense to you! I'll try to update later when I have my iMac to fiddle w/.