So let's say I have a clip with a filter on it (contrast, threshold, whatever) I want an attribute of the filter to change according to the trend of the sound that accompanies the clip.

So if I have a clip of a man talking, If he isn't speaking, there is no effect, if he is whispering, there is a little effect, if he's yelling, the effect is very pronounced.

How can I make this happen? What software do I need? I have FCP and Motion. But I can get access to other things through my school if need be.

You may want to check out After Effects.

With the 'Turn Amplitude Into Keyframes' (I think it's called) setting, you get a number, I believe 0-1 or 0-100, of percentage of max volume. You can then tell any setting on any effect to use this value. For example, the sound clip of a flickering light (zzzt-zzzzt) could change the opacity of the video layer, giving the appearance of the light flickering, set by the noise.

C

Good advice, or, really, you could just look at the waveform and do the same thing by hand in any application. Might be a bit slow, but shouldn't be too hard.

Another good resource for AE.
http://www.videocopilot.net/tutorials.html
Although I would do it by hand if the clip wasn't too long, like over 10 minutes.