Thanks for the feedback.
Question: How do you suppress the "natural sounds?" as suggested by a few of you.
Rob, in an editing program you'll find the image and audio tracks are separated... usually there will be a function (button, slider, command, etc) that either allows you to either mute or vary the audio levels. You can kill off the live audio by muting it, then add a separate audio track (music/voiceover) to fit your action at full volume. You can switch between the 2 (or more) tracks, or have them overlaying each other if you want to maintain some of the original audio at any point.
In iMovie it looks like you select the 2nd last centre button (speaker icon) and use that to alter the volume in the pop-up window. I don't know if iMovie allows you to add additional tracks, it probably does somehow by looking at the controls in that pop-up panel.
If you're on a Macbook, iMovie is a good start since its free, but I never got the hang of it. FCPro 7 is a reasonably expensive "Pro" app but very good, used extensively at my work. Apple, in their wisdom, got rid of it a year ago and created Final Cut X
(10) @ $300.00, a souped up version of iMovie.... the pro's whined loudly and Apple are relicensing FCP7 for existing users, since X just wasn't mature enough. I think you could only get X nowadays.
Another freebie (for windows) is Lightworks
which can be upgraded to "pro" very cheaply... haven't looked at it much but its a bit of a weird one to me.
I bought into Sony Vegas
a few years back (Windows) not too pricey and does all I'll ever need for home editing.. but there's also a "Pro" version of that too.
And then there's Avid (Mac/Linux)... no need to go there.. purely pro.
I think Adobe Premiere is coming along too...haven't used it, part of the Adobe creative suite.... not cheap.
With editing you get what you pay for... but prices have dropped enormously so if you favour Mac, then probably jump straight to FCX if you really want to play.
But there's lots a talented person can do with iMovie... here's one of my colleagues (an animation director) Youtube channel
of ride movies, all this on iMovie on a laptop.