Recording Computer Input for audio and syncing it with video?

Okay I'm trying to figure out a way to do this....
I make hiphop music and I wanted to make a video with me creatin a song from scratch. Issue is, the sounds of me hitting my drum machine come out clearer than the finished product from my speakers X_x
So I'm trying to figure out a solution....
There's videos (links below) of people making instrumentals where the computer audio, im guessing the input from whatever program they are working on, is synced with the video of them making the track....
How exactly would I do this on a mac?
examples:
http://www.youtube.com/watch?v=lVCO1Mct4Ck&feature=channel
http://www.youtube.com/watch?v=tRrSdPHflm0&feature=fvsr @ 6:57
http://www.youtube.com/watch?v=2P1W3xjUio8&feature=related
any ideas?
I have imovie and FCP